[ELFYAML] Support arm32 R_ARM_V4BX relocation format

[zatrazz]

Summary

ARM32 ELF R_ARM_V4BX relocation format is a special relocation type
that records the location of an ARMv4t BX instruction to enable a
static linker to generate ARMv4 compatible instructions. This
relocation does not contain a reference symbol.

This patch enabled its creation by removing the requeriment of a
relocation symbol target in ELFState<ELFT>::writeSectionContent.

[shankarke]

Add a test.

[zatrazz]

I will add a test and repost, thanks.

tools/yaml2obj/yaml2elf.cpp

ELFState<ELFT>::writeSectionContent(Elf_Shdr &SHeader,
   bool IsRela = Section.Type == llvm::ELF::SHT_RELA;
   SHeader.sh_entsize = IsRela ? sizeof(Elf_Rela) : sizeof(Elf_Rel);
   SHeader.sh_size = SHeader.sh_entsize * Section.Relocations.size();
 
   auto &OS = CBA.getOSAndAlignedOffset(SHeader.sh_offset);
 
   for (const auto &Rel : Section.Relocations) {
     unsigned SymIdx;
-    if (SymN2I.lookup(Rel.Symbol, SymIdx)) {
-      errs() << "error: Unknown symbol referenced: '" << Rel.Symbol
-             << "' at YAML relocation.\n";
-      return false;
-    }
+    // Some special relocation, R_ARM_v4BX for instance, does not have
+    // an external reference.
+    SymN2I.lookup(Rel.Symbol, SymIdx);
 
     if (IsRela) {
       Elf_Rela REntry;
       zero(REntry);
       REntry.r_offset = Rel.Offset;
       REntry.r_addend = Rel.Addend;
       REntry.setSymbolAndType(SymIdx, Rel.Type, isMips64EL(Doc));
       OS.write((const char *)&REntry, sizeof(REntry));