Replace .mips_hack_stocg with ".set micromips" and ".set nomicromips".

This matches what gnu as does and implementing this is easier than arguing
about it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199181 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/MC/MCStreamer.h

@@ -71,6 +71,9 @@ protected:
 public:
   virtual ~MCTargetStreamer();
   void setStreamer(MCStreamer *S) { Streamer = S; }
+
+  // Allow a target to add behavior to the EmitLabel of MCStreamer.
+  virtual void emitLabel(MCSymbol *Symbol);
 };
 
 // FIXME: declared here because it is used from