{arm64|x86_64}cpuid.pl: add special 16-byte case to OPENSSL_memcmp.

OPENSSL_memcmp is a must in GCM decrypt and general-purpose loop takes
quite a portion of execution time for short inputs, more than GHASH for
few-byte inputs according to profiler. Special 16-byte case takes it off
top five list in profiler output.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6312)