sha2-ppc: better optimization for POWER9

* cipher/sha256-ppc.c: Change to use vector registers, generate
POWER8 and POWER9 from same code with help of 'target' and
'optimize' attribute.
* cipher/sha512-ppc.c: Likewise.
* configure.ac (gcry_cv_gcc_attribute_optimize)
(gcry_cv_gcc_attribute_ppc_target): New.

Benchmark on POWER9:

Before:

|  nanosecs/byte   mebibytes/sec   cycles/byte

SHA256 | 5.22 ns/B 182.8 MiB/s 12.00 c/B
SHA512 | 3.53 ns/B 269.9 MiB/s 8.13 c/B

After (sha256 ~12% faster, sha512 ~19% faster):

|  nanosecs/byte   mebibytes/sec   cycles/byte

SHA256 | 4.65 ns/B 204.9 MiB/s 10.71 c/B
SHA512 | 2.97 ns/B 321.1 MiB/s 6.83 c/B

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>