cmd/go: fix data race on cgoLibGccFile Fixes issue 4426.
cmd/go: fix data race on cgoLibGccFile
Fixes issue 4426.

R=dvyukov
CC=golang-dev
http://codereview.appspot.com/6851099

tv42/old-go,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,gnanderson/go,gnanderson/go,apparentlymart/golang,tv42/old-go,apparentlymart/golang,gnanderson/go,gnanderson/go,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,pombredanne/go-deleteme,dsriram/golang,tv42/old-go,dsriram/golang,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang,dsriram/golang,apparentlymart/golang,dsriram/golang