[yaml2obj] Add default sh_entsize for dynamic sections

[paulsemel]

Summary

Dynamic section holds a table, so the sh_entsize might be set. As the dynamic section entry size never changes, we can default it to the size of a dynamic entry.

[ruiu]

LGTM

llvm/trunk/test/tools/yaml2obj/dynamic-section.yaml

+# Ensures that dynamic section has sh_entsize correctly set
+# RUN: yaml2obj %s -o %t
+# RUN: llvm-readobj -sections %t | FileCheck %s --check-prefix=SECTION
+
+!ELF
+FileHeader:
+  Class:           ELFCLASS64
+  Data:            ELFDATA2LSB
+  Type:            ET_DYN
+  Machine:         EM_X86_64
+Sections:
+  - Name: .dynamic
+    Type: SHT_DYNAMIC
+    Flags: [ SHF_ALLOC, SHF_WRITE ]
+
+# SECTION: Name: .dynamic
+# SECTION: EntrySize: 16

llvm/trunk/tools/yaml2obj/yaml2elf.cpp

 class ELFState {
   typedef typename ELFT::Ehdr Elf_Ehdr;
   typedef typename ELFT::Phdr Elf_Phdr;
   typedef typename ELFT::Shdr Elf_Shdr;
   typedef typename ELFT::Sym Elf_Sym;
   typedef typename ELFT::Rel Elf_Rel;
   typedef typename ELFT::Rela Elf_Rela;
   typedef typename ELFT::Relr Elf_Relr;
+  typedef typename ELFT::Dyn Elf_Dyn;
 
   enum class SymtabType { Static, Dynamic };
 
   /// The future ".strtab" section.
   StringTableBuilder DotStrtab{StringTableBuilder::ELF};
 
   /// The future ".shstrtab" section.
   StringTableBuilder DotShStrtab{StringTableBuilder::ELF};
assert(Section.Size >= Section.Content.binary_size() &&
          "Section size and section content are inconsistent");
   raw_ostream &OS =
       CBA.getOSAndAlignedOffset(SHeader.sh_offset, SHeader.sh_addralign);
   Section.Content.writeAsBinary(OS);
   for (auto i = Section.Content.binary_size(); i < Section.Size; ++i)
     OS.write(0);
   if (Section.Type == llvm::ELF::SHT_RELR)
     SHeader.sh_entsize = sizeof(Elf_Relr);
+  else if (Section.Type == llvm::ELF::SHT_DYNAMIC)
+    SHeader.sh_entsize = sizeof(Elf_Dyn);
   else
     SHeader.sh_entsize = 0;
   SHeader.sh_size = Section.Size;
 }
 
 static bool isMips64EL(const ELFYAML::Object &Doc) {
   return Doc.Header.Machine == ELFYAML::ELF_EM(llvm::ELF::EM_MIPS) &&
          Doc.Header.Class == ELFYAML::ELF_ELFCLASS(ELF::ELFCLASS64) &&