sha/asm/sha512p8-ppc.pl: improve POWER9 performance by ~10%.

Biggest part, ~7%, of improvement resulted from omitting constants'
table index increment in each round. And minor part from rescheduling
instructions. Apparently POWER9 (and POWER8) manage to dispatch
instructions more efficiently if they are laid down as if they have
no latency...

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6406)