cmd/go: do not print GCC environment variables on Plan 9
cmd/go: do not print GCC environment variables on Plan 9

R=seed, rminnich, r, minux.ma
CC=golang-dev
https://codereview.appspot.com/7397059

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