cmd/go: pass signals forward during "go tool"

This way, if a SIGINT is sent to the go command,
it is forwarded on to the underlying tool.

Otherwise trying to use os.Process.Signal to kill
"go tool compile" only kills the "go tool" not the "compile".

Change-Id: Iac7cd4f06096469f5e76164df813a379c0da3822
Reviewed-on: https://go-review.googlesource.com/c/go/+/282312


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>