aarch64: Optimized implementation of memrchr

Considering the excellent performance of memchr.S on glibc 2.30, the
same algorithm is used to find chrin. Compared to memrchr.c, this
method with memrchr.S achieves an average performance improvement
of 58% based on benchtest and its extension cases.

Checked on aarch64-linux-gnu.

Reviewed-by: Wilco Dijkstra <Wilco.Dijkstra@arm.com>