crypto/sha512: add BE support to PPC64 asm implementation

This adds big endian support for the assembly implementation of
sha512. There was a recent request to do this for sha256 for
AIX users; for completeness, the same is being done for sha512.
The majority of the code is common between big and little
endian with a few differences controlled by ifdefs: with LE
the generation of a mask is needed along with VPERM instructions
to put bytes in the correct order; some VPERMs need the V
registers in a different order.

name        old time/op    new time/op     delta
Hash8Bytes    1.02µs ± 0%     0.38µs ± 0%   -62.68%
Hash1K        7.01µs ± 0%     2.43µs ± 0%   -65.42%
Hash8K        50.2µs ± 0%     14.6µs ± 0%   -70.89%

Updates #50785

Change-Id: I739b5e7c07b22b5748af11ca781e82ac67adb4f7
Reviewed-on: https://go-review.googlesource.com/c/go/+/388654


Reviewed-by: Cherry Mui <cherryyz@google.com>
Trust: Lynn Boger <laboger@linux.vnet.ibm.com>
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gopher Robot <gobot@golang.org>