Use the cpu-features NDK library for detecting the number of cores in WelsThreadLib

On arm, the exact same detection is done in WelsCPUFeatureDetect,
but in the x86 version of that function we use x86 cpuid for getting
the core count, and this is not available on all processors. For the
case when cpuid can't tell the core count, use the NDK function as
higher level API.

The thread lib itself doesn't build properly on android yet, but will
do so soon.
Merge pull request #434 from mstorsjo/threadlib-core-count-android

Use the cpu-features NDK library for detecting the number of cores in WelsThreadLib