go/build: use GCCGOTOOLDIR to find gccgo tools

Add the environment variable GCCGOTOOLDIR to permit overriding the default
directory where tools like cgo are found when building with gccgo.

This will be used by the cmd/go tests in a future CL.

Change-Id: I07855bd3f64cced1351183c0408ffa29741b7c3e
Reviewed-on: https://go-review.googlesource.com/45694
Reviewed-by: Than McIntosh <thanm@google.com>
3 files changed