ossl: Add GCM support on powerpc64/powerpc64le (POWER8+)

Separate ossl's existing AES-NI GCM implementation into a common
ossl_aes_gcm.c and add conditionals to switch between OpenSSL's AES-NI
and POWER8 GCM routines depending on the architecture. Since the
existing AVX-512 implementation is less agnostic, move it into a
separate ossl_aes_gcm_avx512.c.

Reviewed by: markj
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D44274