vpx: enable runtime cpu detection

Older PCs may not support all instruction set extensions. If Ring was
built on a newer PC, this can cause a SIGILL crash when running on an
older PC.

Keep it disabled on Android and iOS, as they have no CPU detection
method available.

For example, the Core i5-2520M (Lenovo L520) does not support avx2.

Change-Id: I5bf6507ef174e18202fc6867772cd0a6be385388
Reviewed-by: Anthony Léonard <anthony.leonard@savoirfairelinux.com>