Unify the interface for the different variants of WelsCPUFeatureDetect

The caller of the function should not need to know exactly which
implementation of it is being used.

For the variants that don't support detecting the number of cores,
the pNumberOfLogicProcessors parameter can be left untouched
and the caller will use a higher level API for finding it out.

This simplifies all the calling code, and simplifies adding
more implementations of cpu feature detection.
Add cpu feature detection for generic arm/linux

For platforms without runtime detection, assume whoever built it
with HAVE_NEON actually wanted it.
Merge pull request #402 from mstorsjo/unify-cpu-feature-detect

Unify the interface for the different variants of WelsCPUFeatureDetect