libc/amd64: rewrite memrchr() baseline impl. to read the string from the back

This ensures O(1) behaviour if the character is a constant offset
from the end of the string, regardless of how long the string is.

Reported by: Mikael Simonsson <m@mikaelsimonsson.com>
Reviewed by: benni
PR: 288321
MFC after: 1 month