As of CL 163747, we can write arbitrary length strings
in assembly DATA instructions.

Make use of it here to improve readability.

Change-Id: I556279ca893f527874e3b26112c43573834ccd9c
Reviewed-on: https://go-review.googlesource.com/c/go/+/167386
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>