vc4: Use NEON to speed up utile loads on Pi2.

vc4: Use NEON to speed up utile loads on Pi2.

We had a lot of memcpy call overhead because gpu_stride wasn't being
inlined.  But if you split out the stride==8 and stride==16 cases like
this code does while still using memcpy, you'd no longer have glibc's
NEON memcpy applied at which point we'd be doing 16 uncached reads
instead of 64/(NEON memcpy granularity), for about a 30% performance
hit.  By hand writing the assembly, we can get a whole cacheline
loaded at a time.

Unfortunately, NEON intrinsics turned out to be unusable -- they
didn't have the vldm instruction available.

Note that, for now, the NEON code is only enabled when building for ARMv7
(Pi 2+).  We may want to do runtime detection for the Raspbian case, in
the future.

Improves 1024x1024 GetTexImage by 208.256% +/- 7.07029% (n=10).