[BOLT] Adjust Shdr alignment

[urnathan]

Summary

Shdr's are not necessarily size 2^n -- that's an accident of ELF64_Shdr's contents.
There is no reason to align to that boundary if they are.

This fixes an assert fail for ELF32 (which requires more patches)

[maksfb]

LGTM. Thanks.

bolt/lib/Rewrite/RewriteInstance.cpp

void RewriteInstance::patchELFSectionHeaderTable(ELFObjectFile<ELFT> *File) {
   std::vector<ELFShdrTy> OutputSections =
       getOutputSections(File, NewSectionIndex);
   LLVM_DEBUG(
     dbgs() << "BOLT-DEBUG: old to new section index mapping:\n";
     for (uint64_t I = 0; I < NewSectionIndex.size(); ++I)
       dbgs() << "  " << I << " -> " << NewSectionIndex[I] << '\n';
   );
 
-  // Align starting address for section header table.
+  // Align starting address for section header table. There's no architecutal
+  // need to align this, it is just for pleasant human readability.
   uint64_t SHTOffset = OS.tell();
-  SHTOffset = appendPadding(OS, SHTOffset, sizeof(ELFShdrTy));
+  SHTOffset = appendPadding(OS, SHTOffset, 16);
 
   // Write all section header entries while patching section references.
   for (ELFShdrTy &Section : OutputSections) {
     Section.sh_link = NewSectionIndex[Section.sh_link];
     if (Section.sh_type == ELF::SHT_REL || Section.sh_type == ELF::SHT_RELA) {
       if (Section.sh_info)
         Section.sh_info = NewSectionIndex[Section.sh_info];
     }