cmd/go: for gccgo, look for tool build ID before hashing entire file
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 <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Jay Conrod <f3fadaedc813344da5ec13b41e26596889cc5baa@google.com>

nawawi/go,CAFxX/go,akutz/go,nawawi/go,akutz/go,akutz/go,CAFxX/go,nawawi/go,cloudflare/go,nawawi/go,golang/go,CAFxX/go,cloudflare/go,akutz/go,akutz/go,CAFxX/go,cloudflare/go,cloudflare/go,golang/go,akutz/go,cloudflare/go,golang/go,akutz/go,nawawi/go,cloudflare/go,golang/go,golang/go,akutz/go,nawawi/go,CAFxX/go,CAFxX/go,nawawi/go,golang/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,cloudflare/go,golang/go,CAFxX/go