Though this doesn't mean that masm becomes supported, the script is
still provided on don't-ask-in-case-of-doubt-use-nasm basis.
See RT#3650 for background.

(Imported from upstream's 2f8d82d641)

The data_word changes were already fixed with our
3e700bb3e812730f60bec529a14c0d35ea9665aa, but best to avoid diverging there.

Change-Id: Iab5455534e8bd632fb2b247ff792d411b105f17a
Reviewed-on: https://boringssl-review.googlesource.com/3581
Reviewed-by: Adam Langley <agl@google.com>

We were building the NASM flavor with MASM which is why it didn't work. Get the
MASM output working: cpuid and cmove are not available in MASM unless the file
declares .686. Also work around MASM rejecting a very long line in SHA-256.

The follow-up change will get the NASM flavor working. We should probably use
that one as it's documented as supported upstream. But let's make this one
functional too.

Change-Id: Ica69cc042a7250c7bc9ba9325caab597cd4ce616
Reviewed-on: https://boringssl-review.googlesource.com/2091
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.)