Although x86masm.pl exists, upstream's documentation suggest only x86nasm.pl is
supported. Yasm seems to handle it fine with a small change.

Change-Id: Ia77be57c6b743527225924b2b398f2f07a084a7f
Reviewed-on: https://boringssl-review.googlesource.com/2092
Reviewed-by: Adam Langley <agl@google.com>

Initial fork from f2d678e6e8 (1.0.2 beta).

(This change contains substantial changes from the original and
effectively starts a new history.)