misc/cgo/test/issue9400: fix to build with gccgo

This doesn't test much with gccgo, but at least it builds now, and the
test does, unsurprisingly, pass.  A proper test would require adding
assembly files in GCC syntax for all platforms that gccgo supports,
which would be infeasible.

Also added copyright headers to the asm files.

Change-Id: Icea5af29d7d521a0681506ddb617a79705b76d33
Reviewed-on: https://go-review.googlesource.com/9417


Reviewed-by: Minux Ma <minux@golang.org>