*/string.3,arch.7,environ.7: add cross-references to simd(7)

With the new simd-dispatch framework documented in simd(7),
add cross-references to the new man pages to appropriate places.

Sponsored by: The FreeBSD Foundation
Approved by: emaste
MFC to: stable/14
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D41697