devel/isa-l_crypto: fix packaging on powerpc64le, enable powerpc64

Upstream 2.26 moved aes_cbc.h and aes_gcm.h into the AES module's
install list, and the AES module is only built on x86_64 and aarch64.
Other architectures use the generic base implementations, so the two
headers are not staged and check-plist fails. Gate them in pkg-plist.

powerpc64 builds the same generic code: both powerpc64 and powerpc64le
pass the upstream test suite (29/29, including the FIPS self-tests),
so widen ONLY_FOR_ARCHS.

Also correct ONLY_FOR_ARCHS_REASON: no isa-l_crypto release ever had
powerpc64le-specific code; all releases fall back to the generic C
implementations on ppc64*.