cmd/go: fix failing gccgo cases in TestScript/build_overlay

The 'go install' command does not support the -gccgo flag.
(I'm not sure why, but it doesn't.)

gccgo also uses system-native assembly syntax instead of cmd/compile's
Plan 9 derivative. I've added an assembly file that seems to work on
Linux, but I haven't tested it on other platforms; if it fails on
other platforms, we can refine the test as needed.

Fixes #42688

Change-Id: I0693a6a9eb58975f20cdc4160ef5f9a948563c88
Reviewed-on: https://go-review.googlesource.com/c/go/+/270978


Trust: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>