Use cmpb which compares each byte in two 64 bit values and
for each matching byte places 0xff in the target and 0x00
otherwise.

A simple hash_name microbenchmark:

http://ozlabs.org/~anton/junkcode/hash_name_bench.c

shows this version to be 10-20% faster than running the x86
version on POWER8, depending on the length.
Signed-off-by: NAnton Blanchard <anton@samba.org>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>