cmd/go: don't add C compiler ID to hash for standard library
No test because a real test requires installing two different compilers.
For #40042
For #47251
Change-Id: Iefddd67830d242a119378b7ce20be481904806e4
Reviewed-on: https://go-review.googlesource.com/c/go/+/335409
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Bryan C. Mills <bcmills@google.com> Reviewed-by: Jay Conrod <jayconrod@google.com>