arch: fix neon feature detection

The fread function returns the number of elements read, not the number
of bytes.

Introduced by 1cef8339f7a2a503327c20c014e696c91d36281e , after giant.

Add tests for protection against similar regressions.

http://tracker.ceph.com/issues/10185 Fixes: #10185

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