[clang] [llvm] [PowerPC] Support local-dynamic TLS relocation on AIX (PR #66316)

[Amy Kwan via cfe-commits]

================
@@ -116,6 +116,10 @@ std::pair<uint8_t, uint8_t> PPCXCOFFObjectWriter::getRelocTypeAndSignSize(
       return {XCOFF::RelocationType::R_TLS_IE, SignAndSizeForFKData};
     case MCSymbolRefExpr::VK_PPC_AIX_TLSLE:
       return {XCOFF::RelocationType::R_TLS_LE, SignAndSizeForFKData};
+    case MCSymbolRefExpr::VK_PPC_AIX_TLSLD:
----------------
amy-kwan wrote:

I am wondering, maybe the `IsSigned` does not matter in this situation due to the above comment?
```
  // People from AIX OS team says AIX link editor does not care about
  // the sign bit in the relocation entry "most" of the time.
  // The system assembler seems to set the sign bit on relocation entry
  // based on similar property of IsPCRel. So we will do the same here.
  // TODO: More investigation on how assembler decides to set the sign
  // bit, and we might want to match that.
  const uint8_t EncodedSignednessIndicator = IsPCRel ? SignBitMask : 0u;
```

https://github.com/llvm/llvm-project/pull/66316