Improve readability and correctness of the OS specific libunwind bits.

[ed]

Summary

All of the access to __exidx_*, dl_iterate_phdr(), etc. is specific to
the findUnwindSections() function. Right now all of the includes and
declarations related to them are scattered throughout the source file.
For example, for <link.h>, we have a full list of operating systems
guarding the #include, even though the code that uses dl_iterate_phdr()
miraculously doesn't use the same list.

Change the code so that findUnwindSections() is preceded by a block of
#ifdefs that share the same structure as the function itself. First
comes all of the macOS specific bits, followed by bare-metal ARM,
followed by ELF EHABI + DWARF.

This actually allows us to build a copy of libunwind without any
specific ifdefs for NetBSD, CloudABI, etc. It likely also unbreaks the
build of libunwind on FreeBSD/armv6, though I can't confirm.

[compnerd]

Not sure if I see the movement as being a real improvement. However, it is no worse than before, and not having the explicit list of targets is a huge win.

[bsdjhb]

This broke the build on FreeBSD it seems because the include of <link.h> is now done inside of the open 'libunwind' namespace. <link.h> pulls in <sys/types.h> causing types like pid_t to be defined in the libunwind namespace which then fails to build:

In file included from /home/john/work/git/llvm/runtimes/libunwind/src/libunwind.cpp:28:
In file included from /home/john/work/git/llvm/runtimes/libunwind/src/UnwindCursor.hpp:20:
In file included from /usr/include/pthread.h:46:
/usr/include/sched.h:236:24: error: C++ requires a type specifier for all
      declarations
int     sched_getparam(pid_t, struct sched_param *);
                       ^
/usr/include/sched.h:237:28: error: unknown type name 'pid_t'; did you mean
      'libunwind::pid_t'?
int     sched_getscheduler(pid_t);
                           ^
/usr/include/sys/types.h:183:18: note: 'libunwind::pid_t' declared here
typedef __pid_t         pid_t;          /* process id */
                        ^

While we could close the namespace inside of each of the #ifdef blocks, I wonder if it wouldn't be cleaner to move this back up to where it was originally before the namespace opens but maybe inside a '#ifndef APPLE'?

[compnerd]

@bsdjhb shouldn't link.h be includable in a C++ header without worrying about the namespacing? But, moving the declarations is certainly reasonable.

src/AddressSpace.hpp

 }
 #endif
 
 #include "libunwind.h"
 #include "config.h"
 #include "dwarf2.h"
 #include "Registers.hpp"
 
-#if _LIBUNWIND_ARM_EHABI
-#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 char __exidx_start;
-extern char __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
-// macro is not available on some systems (e.g., FreeBSD). On these
-// systems the data structures are just called Elf_XXX. Define ElfW()
-// locally.
-#if !defined(ElfW)
-#define ElfW(type) Elf_##type
-#endif
-#include "EHHeaderParser.hpp"
-#endif
-#endif
-
 namespace libunwind {
 
 /// Used by findUnwindSections() to return info about needed sections.
 struct UnwindInfoSections {
 #if _LIBUNWIND_SUPPORT_DWARF_UNWIND || _LIBUNWIND_SUPPORT_DWARF_INDEX ||       \
     _LIBUNWIND_SUPPORT_COMPACT_UNWIND
   // No dso_base for ARM EHABI.
   uintptr_t       dso_base;
LocalAddressSpace::getEncodedP(pint_t &addr, pint_t end, uint8_t encoding,
 
   if (encoding & DW_EH_PE_indirect)
     result = getP(result);
 
   return result;
 }
 
 #ifdef __APPLE__ 
+
   struct dyld_unwind_sections
   {
     const struct mach_header*   mh;
     const void*                 dwarf_section;
     uintptr_t                   dwarf_section_length;
     const void*                 compact_unwind_section;
     uintptr_t                   compact_unwind_section_length;
   };
#endif
 
       if (!info->compact_unwind_section) {
         info->compact_unwind_section_length = 0;
       }
 
       return true;
     }
   #endif
+
+#elif _LIBUNWIND_ARM_EHABI && 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 char __exidx_start;
+extern char __exidx_end;
+
+#elif _LIBUNWIND_ARM_EHABI || _LIBUNWIND_SUPPORT_DWARF_UNWIND
+
+// ELF-based systems may use dl_iterate_phdr() to access sections
+// containing unwinding information. The ElfW() macro for pointer-size
+// independent ELF header traversal is not provided by <link.h> on some
+// systems (e.g., FreeBSD). On these systems the data structures are
+// just called Elf_XXX. Define ElfW() locally.
+#include <link.h>
+#if !defined(ElfW)
+#define ElfW(type) Elf_##type
+#endif
+
+#if _LIBUNWIND_SUPPORT_DWARF_UNWIND
+#include "EHHeaderParser.hpp"
+#endif
+
 #endif
 
 inline bool LocalAddressSpace::findUnwindSections(pint_t targetAddr,
                                                   UnwindInfoSections &info) {
 #ifdef __APPLE__
   dyld_unwind_sections dyldInfo;
   if (_dyld_find_unwind_sections((void *)targetAddr, &dyldInfo)) {
     info.dso_base                      = (uintptr_t)dyldInfo.mh;