[llvm] [RISCV] Vendor Relocations for Xqci extension (PR #135400)

[Fangrui Song via llvm-commits]

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@@ -1435,6 +1435,21 @@ void ELFObjectWriter::recordRelocation(MCAssembler &Asm,
                         : C;
   FixedValue = usesRela(TO, FixupSection) ? 0 : Addend;
 
+  // Some RISC-V relocations need a marker relocation that appears before the
+  // relocation in question.
+  unsigned PreRelocType;
+  MCSymbol *PreRelocSymbol;
+  uint64_t PreRelocAddend;
+  if (Backend.fixupNeedsMarkerELFRelocation(Asm, *Fragment, Fixup, PreRelocType,
----------------
MaskRay wrote:

Cc @jrtc27  who introduced `evaluateTargetFixup`

Currently, the MCFixupKindInfo::FKF_IsTarget code path in MCAssembler.cpp skips the common code paths (getAddSym/getSubSym), because PCREL_LO12[IS] don't need them.

Now evaluateTargetFixup is reused by QUIC relocations, leading to some duplication. I think we can make evaluateTargetFixup return more information so that  QUIC relocations keep reusing the common code path.

```cpp
  if (FixupFlags & MCFixupKindInfo::FKF_IsTarget) {
    IsResolved =
        getBackend().evaluateTargetFixup(*this, Fixup, DF, Target, STI, Value);
  } else {
    const MCSymbol *Add = Target.getAddSym(); 7 refs
    const MCSymbol *Sub = Target.getSubSym();
```

The latter half of the duplication that adds a R_RISCV_VENDOR relocation can probably be refactored to reuse `Type = TargetObjectWriter->getRelocType(Ctx, Target, Fixup, IsPCRel);`

https://github.com/llvm/llvm-project/pull/135400