gc: fix mkopnames and color grep interaction

Fixes #406.

R=adg
CC=golang-dev
https://golang.org/cl/207053

src/cmd/gc/mkopnames

@@ -3,6 +3,10 @@
 # Use of this source code is governed by a BSD-style
 # license that can be found in the LICENSE file.
 
+# Disable colored grep if user has it set to --color=always.
+# (Arguably user error.)
+export GREP_OPTIONS=""
+
 echo '// auto generated by mkopnames'
 echo 'static char*'
 echo 'opnames[] = '