lib/libc/amd64/string: add timingsafe_bcmp(3) scalar, baseline implementations

Very straightforward and similar to memcmp(3). The code has
been written to use only instructions specified as having
data operand independent timing by Intel.

Sponsored by: The FreeBSD Foundation
Approved by: security (cperciva)
Differential Revision: https://reviews.freebsd.org/D41673