Clang (3.6, at least) doesn't like .arch when its internal as is used.
Instead, one has to pass -march=armv8-a+crypto on the command line.

Change-Id: Ifc5b57fbebd0eb53658481b0a0c111e808c81d93
Reviewed-on: https://boringssl-review.googlesource.com/4411
Reviewed-by: Adam Langley <agl@google.com>