simd.7: add scalar memchr() for RISC-V to manpage
MFC after: 1 month
MFC to: stable/15
Approved by: mhorne, markj (mentor)
Sponsored by: Google LLC (GSoC 2024)
Differential Revision: https://reviews.freebsd.org/D46023
(cherry picked from commit d2c23f5953a823e01f0c6418b0f98c119f145e20)