AArch64: Improve strrchr
Use shrn for narrowing the mask which simplifies code and speeds up small
strings. Unroll the first search loop to improve performance on large
strings.
Reviewed-by: 
Szabolcs Nagy <szabolcs.nagy@arm.com>
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Use shrn for narrowing the mask which simplifies code and speeds up small
strings. Unroll the first search loop to improve performance on large
strings.
Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>