read/xcoff: add support for relocation entries.

[EsmeYi]

No description provided.

[philipc]

Can you explain your reasoning for R_TLS being absolute? I'm not sure about it. It might be okay, and there may not be a better alternative.

[EsmeYi]

It seems treated as absolute in llvm

  if (Type == XCOFF::RelocationType::R_POS ||
      Type == XCOFF::RelocationType::R_TLS)
    // The FixedValue should be symbol's virtual address in this object file
    // plus any constant value that we might get.
    FixedValue = getVirtualAddress(SymA, SymASec) + Target.getConstant();