[MC][AMDGPU] Support .reloc BFD_RELOC_{NONE,32,64}

Emitting BFD_RELOC_* reloc directives can cause internal errors on AMDGPU

Author: EthanLuisMcDonough

llvm/lib/Target/AMDGPU/MCTargetDesc/AMDGPUAsmBackend.cpp

@@ -163,12 +163,17 @@ void AMDGPUAsmBackend::applyFixup(const MCAssembler &Asm, const MCFixup &Fixup,
 
 std::optional<MCFixupKind>
 AMDGPUAsmBackend::getFixupKind(StringRef Name) const {
-  return StringSwitch<std::optional<MCFixupKind>>(Name)
-#define ELF_RELOC(Name, Value)                                                 \
-  .Case(#Name, MCFixupKind(FirstLiteralRelocationKind + Value))
+  auto Type = StringSwitch<unsigned>(Name)
+#define ELF_RELOC(Name, Value) .Case(#Name, Value)
 #include "llvm/BinaryFormat/ELFRelocs/AMDGPU.def"
 #undef ELF_RELOC
-      .Default(std::nullopt);
+                  .Case("BFD_RELOC_NONE", ELF::R_AMDGPU_NONE)
+                  .Case("BFD_RELOC_32", ELF::R_AMDGPU_ABS32)
+                  .Case("BFD_RELOC_64", ELF::R_AMDGPU_ABS64)
+                  .Default(-1u);
+  if (Type != -1u)
+    return static_cast<MCFixupKind>(FirstLiteralRelocationKind + Type);
+  return std::nullopt;
 }
 
 const MCFixupKindInfo &AMDGPUAsmBackend::getFixupKindInfo(