For the 1.8 release, go back to invoking the assembler once per .s
file, to avoid the problem in #18225. When the assembler is fixed, the
change to cmd/go/build.go can be rolled back, but the test in
cmd/go/go_test.go should remain.

Fixes #18225.
Update #15680.

Change-Id: Ibff8d0c638536efb50a2b2c280b41399332f4fe4
Reviewed-on: https://go-review.googlesource.com/34284
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>