[RISCV] Support assembling @plt symbol operands

This patch allows symbols appended with @plt to parse and assemble with the
R_RISCV_CALL_PLT relocation.

Differential Revision: https://reviews.llvm.org/D55335
Patch by Lewis Revill.

llvm-svn: 357470
diff --git a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVMCCodeEmitter.cpp b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVMCCodeEmitter.cpp
index 883ec4d9..692398e 100644
--- a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVMCCodeEmitter.cpp
+++ b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVMCCodeEmitter.cpp
@@ -242,6 +242,10 @@
       FixupKind = RISCV::fixup_riscv_call;
       RelaxCandidate = true;
       break;
+    case RISCVMCExpr::VK_RISCV_CALL_PLT:
+      FixupKind = RISCV::fixup_riscv_call_plt;
+      RelaxCandidate = true;
+      break;
     }
   } else if (Kind == MCExpr::SymbolRef &&
              cast<MCSymbolRefExpr>(Expr)->getKind() == MCSymbolRefExpr::VK_None) {