share/man/man7/simd.7: document strncmp amd64 scalar, baseline implementations

Sponsored by: The FreeBSD Foundation
Tested by: developers@, exp-run
Approved by: mjg
MFC after: 1 month
MFC to: stable/14
PR: 275785
Differential Revision: https://reviews.freebsd.org/D42122

(cherry picked from commit f5edd8450e5328c5b88977b4d00e780cdd744dea)