Add a script to generate the ChaCha ARM asm.

Obviously I shouldn't be doing this by hand each time.

Change-Id: I64e3f5ede5c47eddbff3b18172a95becc681b486
Reviewed-on: https://boringssl-review.googlesource.com/3170
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>