[release-branch.go1.14-security] 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.

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>
(cherry picked from commit 4cf399ca38587a6e4a3e85b494cd9a9b4cc53378)
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/955294


Reviewed-by: Katie Hockman <katiehockman@google.com>