provide a NEON version of arm/sgemm

benchmark/sgemm.goto before:
 M= 200, N= 200, K= 200 :     9262.97 MFlops   0.001727 sec
after:
 M= 200, N= 200, K= 200 :    30223.64 MFlops   0.000529 sec

Conveniently the registers are already allocated suitably for vector
operation, so the conversion from vfpv3 was rather straightforward.

Prefetching was left out because it doesn't help Cortex-A76,
only hurts it slightly.

Author: notaz