arch: add support for HW_CAP based neon runtime detection

Rename the files from neon to arm to reflect the fact that it's related
to arm processors and also because NEON was renamed ASIMD later. The
NEON and ASIMD features are mutually exclusive. 32bits binaries will get
NEON and never ASIMD, if they run on ARMv7 or ARMv8. 64bits binaries
will only run on ARMv8 and get ASIMD and never NEON.

The flag remains with _neon and no other flag is introduced since there
is no risk of confusion. Besides people who care usually know NEON but
are not yet aware of the ASIMD renaming. Keeping the _neon name probably
saves some questions.

http://tracker.ceph.com/issues/10197 Fixes: #10197

Signed-off-by: Loic Dachary <ldachary@redhat.com>