Add GHASH for PowerISA 2.0.7.

Reviewed-by: Tim Hudson <tjh@openssl.org>
Engage GHASH for PowerISA 2.0.7.

[and split ppccap.c to ppccap.c and ppc_arch.h]

Reviewed-by: Kurt Roeckx <kurt@openssl.org>
Reviewed-by: Tim Hudson <tjh@openssl.org>