[RISCV] Lower calls through PLT

This patch adds support for generating calls through the procedure
linkage table where required for a given ExternalSymbol or GlobalAddress
callee.

Differential Revision: https://reviews.llvm.org/D55304

llvm-svn: 363686
diff --git a/llvm/lib/Target/RISCV/RISCVMCInstLower.cpp b/llvm/lib/Target/RISCV/RISCVMCInstLower.cpp
index dc35944..b40eaa9 100644
--- a/llvm/lib/Target/RISCV/RISCVMCInstLower.cpp
+++ b/llvm/lib/Target/RISCV/RISCVMCInstLower.cpp
@@ -39,6 +39,9 @@
   case RISCVII::MO_CALL:
     Kind = RISCVMCExpr::VK_RISCV_CALL;
     break;
+  case RISCVII::MO_PLT:
+    Kind = RISCVMCExpr::VK_RISCV_CALL_PLT;
+    break;
   case RISCVII::MO_LO:
     Kind = RISCVMCExpr::VK_RISCV_LO;
     break;