*/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, re (gjb)
MFC to: stable/14, releng/14.0
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D41697

(cherry picked from commit d41afb814612ce005d93d2f8d127dcf6ea751cc4)
(cherry picked from commit 7ec2ae8c7cd4400d5c0db690a1fe43cf85b6c5d3)