[RISCV] Support .option rvc and norvc assembler directives

These directives allow the 'C' (compressed) extension to be enabled/disabled 
within a single file.

Differential Revision: https://reviews.llvm.org/D45864
Patch by Kito Cheng

llvm-svn: 332107
diff --git a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVELFStreamer.cpp b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVELFStreamer.cpp
index 243f8ed..6428b11 100644
--- a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVELFStreamer.cpp
+++ b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVELFStreamer.cpp
@@ -37,3 +37,6 @@
 MCELFStreamer &RISCVTargetELFStreamer::getStreamer() {
   return static_cast<MCELFStreamer &>(Streamer);
 }
+
+void RISCVTargetELFStreamer::emitDirectiveOptionRVC() {}
+void RISCVTargetELFStreamer::emitDirectiveOptionNoRVC() {}