cmd/go: pass resolved CC, GCCGO to cgo

This makes sure the go command and cgo agree about
exactly which compiler is being used.

This issue was reported by RyotaK.

Fixes CVE-2021-3115
Fixes #43783

Change-Id: If171c5c8b2523efb5ea2d957e5ad1380a038149c
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/949416


Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/284780


Run-TryBot: Roland Shoemaker <roland@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Roland Shoemaker <roland@golang.org>