Drop the dependency on dl_unwind_find_exidx().

[ed]

Summary

While porting libunwind over to CloudABI for ARMv6, I observed that this
source file doesn't build, as it depends on dl_unwind_find_exidx(),
which CloudABI's C library was lacking. After I added that function,
I still needed to patch up libunwind to define _Unwind_Ptr.

Taking a step back, I wonder why we need to make use of this function
anyway. The unwinder already has some nice code to use dl_iterate_phdr()
to scan for a PT_GNU_EH_FRAME header. The dl_unwind_find_exidx() does
the same thing, except matching PT_ARM_EXIDX instead. We could also do
that ourselves.

This change gets rid of the dl_unwind_find_exidx() call and extends the
dl_iterate_phdr() loop. This approach has the advantage of getting rid
of some of those OS-specific #ifdefs. This now means that if an
operating system only provides dl_iterate_phdr(), it gets support for
unwinding on all architectures. There is no need to add more stuff, just
to get ARMv6 support.

Note to reviewers:

This change is identical to D28082, except that it now adds the
necessary code to do bounds checking on PT_LOAD. The previous version of
this change lacked this, which didn't cause any problems on CloudABI,
but did break the Linux build bots. This is why I reverted it.

src/AddressSpace.hpp

 #endif
 
 #include "libunwind.h"
 #include "config.h"
 #include "dwarf2.h"
 #include "Registers.hpp"
 
 #if _LIBUNWIND_ARM_EHABI
-#if defined(__FreeBSD__) || defined(__NetBSD__) || defined(__OpenBSD__)
-
-typedef void *_Unwind_Ptr;
-
-#elif defined(__linux__)
-
-typedef long unsigned int *_Unwind_Ptr;
-extern "C" _Unwind_Ptr __gnu_Unwind_Find_exidx(_Unwind_Ptr addr, int *len);
-
-// Emulate the BSD dl_unwind_find_exidx API when on a GNU libdl system.
-#define dl_unwind_find_exidx __gnu_Unwind_Find_exidx
-
-#elif !defined(_LIBUNWIND_IS_BAREMETAL)
-#include <link.h>
-#else // !defined(_LIBUNWIND_IS_BAREMETAL)
-// When statically linked on bare-metal, the symbols for the EH table are looked
-// up without going through the dynamic loader.
 struct EHTEntry {
   uint32_t functionOffset;
   uint32_t unwindOpcodes;
 };
+#if defined(_LIBUNWIND_IS_BAREMETAL)
+// When statically linked on bare-metal, the symbols for the EH table are looked
+// up without going through the dynamic loader.
 extern EHTEntry __exidx_start;
 extern EHTEntry __exidx_end;
+#else
+#include <link.h>
 #endif // !defined(_LIBUNWIND_IS_BAREMETAL)
 #endif // _LIBUNWIND_ARM_EHABI
 
 #if defined(__CloudABI__) || defined(__FreeBSD__) || defined(__Fuchsia__) ||  \
     defined(__linux__) || defined(__NetBSD__)
 #if _LIBUNWIND_SUPPORT_DWARF_UNWIND && _LIBUNWIND_SUPPORT_DWARF_INDEX
 #include <link.h>
 // Macro for machine-independent access to the ELF program headers. This

  #if _LIBUNWIND_SUPPORT_DWARF_UNWIND
     info.dwarf_section                 = (uintptr_t)dyldInfo.dwarf_section;
     info.dwarf_section_length          = dyldInfo.dwarf_section_length;
  #endif
     info.compact_unwind_section        = (uintptr_t)dyldInfo.compact_unwind_section;
     info.compact_unwind_section_length = dyldInfo.compact_unwind_section_length;
     return true;
   }
-#elif _LIBUNWIND_ARM_EHABI
- #ifdef _LIBUNWIND_IS_BAREMETAL
+#elif _LIBUNWIND_ARM_EHABI && defined(_LIBUNWIND_IS_BAREMETAL)
   // Bare metal is statically linked, so no need to ask the dynamic loader
   info.arm_section =        (uintptr_t)(&__exidx_start);
   info.arm_section_length = (uintptr_t)(&__exidx_end - &__exidx_start);
- #else
-  int length = 0;
-  info.arm_section = (uintptr_t) dl_unwind_find_exidx(
-      (_Unwind_Ptr) targetAddr, &length);
-  info.arm_section_length = (uintptr_t)length;
- #endif
   _LIBUNWIND_TRACE_UNWINDING("findUnwindSections: section %X length %x",
                              info.arm_section, info.arm_section_length);
   if (info.arm_section && info.arm_section_length)
     return true;
-#elif _LIBUNWIND_SUPPORT_DWARF_UNWIND
-#if _LIBUNWIND_SUPPORT_DWARF_INDEX
+#elif _LIBUNWIND_ARM_EHABI || _LIBUNWIND_SUPPORT_DWARF_UNWIND
   struct dl_iterate_cb_data {
     LocalAddressSpace *addressSpace;
     UnwindInfoSections *sects;
     uintptr_t targetAddr;
   };
 
   dl_iterate_cb_data cb_data = {this, &info, targetAddr};
   int found = dl_iterate_phdr(
       [](struct dl_phdr_info *pinfo, size_t, void *data) -> int {
         auto cbdata = static_cast<dl_iterate_cb_data *>(data);
-        size_t object_length;
         bool found_obj = false;
         bool found_hdr = false;
 
         assert(cbdata);
         assert(cbdata->sects);
 
         if (cbdata->targetAddr < pinfo->dlpi_addr) {
           return false;
         }
 
 #if !defined(Elf_Half)
         typedef ElfW(Half) Elf_Half;
 #endif
 #if !defined(Elf_Phdr)
         typedef ElfW(Phdr) Elf_Phdr;
 #endif
 
+ #if _LIBUNWIND_SUPPORT_DWARF_UNWIND
+  #if !_LIBUNWIND_SUPPORT_DWARF_INDEX
+   #error "_LIBUNWIND_SUPPORT_DWARF_UNWIND requires _LIBUNWIND_SUPPORT_DWARF_INDEX on this platform."
+  #endif
+        size_t object_length;
         for (Elf_Half i = 0; i < pinfo->dlpi_phnum; i++) {
           const Elf_Phdr *phdr = &pinfo->dlpi_phdr[i];
           if (phdr->p_type == PT_LOAD) {
             uintptr_t begin = pinfo->dlpi_addr + phdr->p_vaddr;
             uintptr_t end = begin + phdr->p_memsz;
             if (cbdata->targetAddr >= begin && cbdata->targetAddr < end) {
               cbdata->sects->dso_base = begin;
               object_length = phdr->p_memsz;
#endif
         }
 
         if (found_obj && found_hdr) {
           cbdata->sects->dwarf_section_length = object_length;
           return true;
         } else {
           return false;
         }
+ #else // _LIBUNWIND_ARM_EHABI
+        for (Elf_Half i = 0; i < pinfo->dlpi_phnum; i++) {
+          const Elf_Phdr *phdr = &pinfo->dlpi_phdr[i];
+          if (phdr->p_type == PT_LOAD) {
+            uintptr_t begin = pinfo->dlpi_addr + phdr->p_vaddr;
+            uintptr_t end = begin + phdr->p_memsz;
+            if (cbdata->targetAddr >= begin && cbdata->targetAddr < end)
+              found_obj = true;
+          } else if (phdr->p_type == PT_ARM_EXIDX) {
+            uintptr_t exidx_start = pinfo->dlpi_addr + phdr->p_vaddr;
+            cbdata->sects->arm_section = exidx_start;
+            cbdata->sects->arm_section_length = phdr->p_memsz /
+                                                sizeof(EHTEntry);
+            found_hdr = true;
+          }
+        }
+        return found_obj && found_hdr;
+ #endif
       },
       &cb_data);
   return static_cast<bool>(found);
-#else
-#error "_LIBUNWIND_SUPPORT_DWARF_UNWIND requires _LIBUNWIND_SUPPORT_DWARF_INDEX on this platform."
-#endif
 #endif
 
   return false;
 }
 
 
 inline bool LocalAddressSpace::findOtherFDE(pint_t targetAddr, pint_t &fde) {
 #ifdef __APPLE__