go/build: add GOOS and GOARCH to name of gccgo pkg directory
go/build: add GOOS and GOARCH to name of gccgo pkg directory

This matches the behaviour of builder.includeArgs in
cmd/go/build.go.

R=golang-dev, minux.ma
CC=golang-dev
https://codereview.appspot.com/14535048

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