gallium: Enable ARM NEON CPU detection.

gallium: Enable ARM NEON CPU detection.

I wrote this code with reference to pixman, though I've only decided to
cover Linux (what I'm testing) and Android (seems obvious enough).  Linux
has getauxval() as a cleaner interface to the /proc entry, but it's more
glibc-specific and I didn't want to add detection for that.

This will be used to enable NEON at runtime on ARMv6 builds of vc4.

v2: Actually initialize the temp vars in the Android path (noticed by
    daniels)
v3: Actually pull in the cpufeatures library (change by robher).
    Use O_CLOEXEC.  Break out of the loop when we find our feature.
v4: Drop VFP code, which was confused about what it was detecting and not
    actually used yet.

Reviewed-by: Grazvydas Ignotas <notasas@gmail.com>