spec: Introduce arches_*

spec: Introduce arches_*

With this commit, all architecture lists that we base feature
enablement decisions on are defined within a few lines of each
other, increasing maintainability.

Additionally, generic architecture lists that appear in the
conditions for multiple features are defined, so that repetition
is reduced.

Note that a few checks (numactl, zfs, ceph) have been changed
from %ifarch to %ifnarch for consistency: while doing so, the
corresponding list of architectures has also been replaced with
the complement of the original one to ensure the overall behavior
would be preserved.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Neal Gompa <ngompa13@gmail.com>