Remove usage of GCC intrinsic and fix bug in alignment computation.

[zturner]

Summary

GCC intrinsics are not available on all compilers, so this converts the usage of __builtin_ffs() to a portable equivalent.

This also seemingly fixes a bug in this code, as __builtin_ffs returns the 1-based index of least-significant 1-bit, but Section::Section() expects the log_2 alignment, which is the 0-based index of the least-significant 1-bit.

[zturner]

Closed by commit rL211669 (authored by @zturner).

source/Plugins/ObjectFile/ELF/ObjectFileELF.cpp

 #include "lldb/Core/Timer.h"
 #include "lldb/Symbol/DWARFCallFrameInfo.h"
 #include "lldb/Symbol/SymbolContext.h"
 #include "lldb/Target/SectionLoadList.h"
 #include "lldb/Target/Target.h"
 #include "lldb/Host/Host.h"
 
 #include "llvm/ADT/PointerUnion.h"
+#include "llvm/Support/MathExtras.h"
 
 #define CASE_AND_STREAM(s, def, width)                  \
     case def: s->Printf("%-*s", width, #def); break;
 
 using namespace lldb;
 using namespace lldb_private;
 using namespace elf;
 using namespace llvm::ELF;
if (!m_sections_ap.get() && ParseSectionHeaders())
                     sect_type = eSectionTypeELFRelocationEntries;
                     break;
                 case SHT_DYNAMIC:
                     assert (sect_type == eSectionTypeOther);
                     sect_type = eSectionTypeELFDynamicLinkInfo;
                     break;
             }
 
+            elf::elf_xword log2align = (header.sh_addralign==0)
+                                        ? 0
+                                        : llvm::Log2_64(header.sh_addralign);
             SectionSP section_sp (new Section(GetModule(),        // Module to which this section belongs.
                                               this,               // ObjectFile to which this section belongs and should read section data from.
                                               SectionIndex(I),    // Section ID.
                                               name,               // Section name.
                                               sect_type,          // Section type.
                                               header.sh_addr,     // VM address.
                                               vm_size,            // VM size in bytes of this section.
                                               header.sh_offset,   // Offset of this section in the file.
                                               file_size,          // Size of the section as found in the file.
-                                              __builtin_ffs(header.sh_addralign), // Alignment of the section
+                                              log2align,          // Alignment of the section
                                               header.sh_flags));  // Flags for this section.
 
             if (is_thread_specific)
                 section_sp->SetIsThreadSpecific (is_thread_specific);
             m_sections_ap->AddSection(section_sp);
         }
     }