cmd/go: include experiments in compiler tool ID

No test because testing this would require building a new toolchain
with a different experiment.

Fixes #26883

Change-Id: Iadd513d0920ef12463006dd2a61e94370dd13f68
Reviewed-on: https://go-review.googlesource.com/128735


Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>