This introduces GC_ERROR to mark an error only issued by the
gc compiler.  GCCGO_ERROR already exists to mark errors only
issued by the gccgo compiler.  Obviously these should be used
sparingly.

bug195.go:9: error: interface contains embedded non-interface
bug195.go:12: error: interface contains embedded non-interface
bug195.go:15: error: interface contains embedded non-interface
bug195.go:18: error: invalid recursive interface
bug195.go:22: error: invalid recursive interface

R=rsc
CC=golang-dev
https://golang.org/cl/2040043