lib/libc/amd64/string: add strrchr scalar, baseline implementation

The baseline implementation is very straightforward, while the scalar
implementation suffers from register pressure and the need to use SWAR
techniques similar to those used for strchr().

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/D42217