test: gccgo and gc print the error on different lines.
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