[JITLink][RISCV] Support relaxable edges without relaxation pass

[jobnoorman]

Summary

Relaxable edges are created unconditionally, even when the relaxation
pass will not run. However, they were not recognized by applyFixup
causing them to not be applied.

To support configurations without the relaxation pass, this patch adds
these relaxable edges to applyFixup:

• CallRelaxable: Can be treated as R_RISCV_CALL
• AlignRelaxable: Can simply be ignored

An alternative could be to unconditionally run the relaxation pass, even
in contexts where shouldAddDefaultTargetPasses returns false. However, I
could imagine there being use cases for disabling relaxation which
wouldn't be possible anymore then.

[StephenFan]

LGTM

llvm/lib/ExecutionEngine/JITLink/ELF_riscv.cpp

case R_RISCV_JAL: {
       uint32_t Imm10_1 = extractBits(Value, 1, 10) << 21;
       uint32_t Imm11 = extractBits(Value, 11, 1) << 20;
       uint32_t Imm19_12 = extractBits(Value, 12, 8) << 12;
       uint32_t RawInstr = *(little32_t *)FixupPtr;
       *(little32_t *)FixupPtr =
           (RawInstr & 0xFFF) | Imm20 | Imm10_1 | Imm11 | Imm19_12;
       break;
     }
+    case CallRelaxable:
+      // Treat as R_RISCV_CALL when the relaxation pass did not run
     case R_RISCV_CALL_PLT:
     case R_RISCV_CALL: {
       int64_t Value = E.getTarget().getAddress() + E.getAddend() - FixupAddress;
       int64_t Hi = Value + 0x800;
       if (LLVM_UNLIKELY(!isInRangeForImm(Hi, 32)))
         return makeTargetOutOfRangeError(G, B, E);
       int32_t Lo = Value & 0xFFF;
       uint32_t RawInstrAuipc = *(little32_t *)FixupPtr;
case R_RISCV_SET32: {
       break;
     }
     case R_RISCV_32_PCREL: {
       int64_t Value = E.getTarget().getAddress() + E.getAddend() - FixupAddress;
       int64_t Word32 = Value & 0xffffffff;
       *(little32_t *)FixupPtr = Word32;
       break;
     }
+    case AlignRelaxable:
+      // Ignore when the relaxation pass did not run
+      break;
     }
     return Error::success();
   }
 };
 
 namespace {
 
 struct SymbolAnchor {