cmd/go: do not install .a files for packages in std

cmd/go: do not install .a files for packages in std

As of CL 450739, we do not need install targets for cgo files when a C
compiler is not present because cgo is not enabled by default.
(Without a C compiler, builds will proceed with cgo disabled.)

Fixes #47257.
Fixes #56888.

Change-Id: I274c50a60b5b1382e291df86a5464da8ad3695a5
Reviewed-on: https://go-review.googlesource.com/c/go/+/452457
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>