[RISCV] Generate address sequences suitable for mcmodel=medium

This patch adds an implementation of a PC-relative addressing sequence to be
used when -mcmodel=medium is specified. With absolute addressing, a 'medium'
codemodel may cause addresses to be out of range. This is because while
'medium' implies a 2 GiB addressing range, this 2 GiB can be at any offset as
opposed to 'small', which implies the first 2 GiB only.

Note that LLVM/Clang currently specifies code models differently to GCC, where
small and medium imply the same functionality as GCC's medlow and medany
respectively.

Differential Revision: https://reviews.llvm.org/D54143
Patch by Lewis Revill.

llvm-svn: 357393
diff --git a/llvm/lib/Target/RISCV/RISCVMCInstLower.cpp b/llvm/lib/Target/RISCV/RISCVMCInstLower.cpp
index 3e50be8..c2c4128 100644
--- a/llvm/lib/Target/RISCV/RISCVMCInstLower.cpp
+++ b/llvm/lib/Target/RISCV/RISCVMCInstLower.cpp
@@ -42,6 +42,12 @@
   case RISCVII::MO_HI:
     Kind = RISCVMCExpr::VK_RISCV_HI;
     break;
+  case RISCVII::MO_PCREL_LO:
+    Kind = RISCVMCExpr::VK_RISCV_PCREL_LO;
+    break;
+  case RISCVII::MO_PCREL_HI:
+    Kind = RISCVMCExpr::VK_RISCV_PCREL_HI;
+    break;
   }
 
   const MCExpr *ME =