lib/libc/amd64/string: add timingsafe_memcmp() assembly implementation

Conceptually very similar to timingsafe_bcmp(), but with comparison
logic inspired by Elijah Stone's fancy memcmp. A baseline (SSE)
implementation was omitted this time as I was not able to get it to
perform adequately. Best I got was 8% over the scalar version for
long inputs, but slower for short inputs.

Sponsored by: The FreeBSD Foundation
Approved by: security (cperciva)
Inspired by: https://github.com/moon-chilled/fancy-memcmp
Differential Revision: https://reviews.freebsd.org/D41696