Avoids confusing errors from the GNU assembler
processing Go assembly source code.

Fixes #19448.

Change-Id: Ic2c68b2521847cca5a3d078a092e5c60ec340840
Reviewed-on: https://go-review.googlesource.com/46423
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>