cmd/go: for gccgo, look for tool build ID before hashing entire file
Also fix the key used to store the ID.
This is a significant speedup in cmd/go run time when using an
unreleased toolchain. For example, the TestGoBuildTestOnly cmd/go test
goes from 15 seconds to 1 second.
Change-Id: Ibfd697d55084db059c6b563f70f71f635e935391
Reviewed-on: https://go-review.googlesource.com/c/go/+/194441
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Jay Conrod <jayconrod@google.com>