cmd/go, go/build: add support for gccgo tooldir

The gccgo toolchain does not put tools (cgo, vet, etc.) in
$GOROOT/pkg/tool, but instead in a directory available at
runtime.GCCGOTOOLDIR.

Update the go/build package and the cmd/go tool to use this tool
directory when using gccgo.

Change-Id: Ib827336ff53601208300aceb77f76c2e1b069859
Reviewed-on: https://go-review.googlesource.com/111097


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>