This adds a new generic vectorization sample to replace hello-neon. Most
importantly, it covers non-Neon options for SIMD. One of the useful
things it shows, for example, is that there's actually no reason to
write SIMD code the way that hello-neon does any more.
This also solves a much simpler problem (small matrix multiplication),
which makes it easier to see how to deal with the SIMD features rather
than figuring out what a FIR filter is.
Finally, this sample benchmarks each of the implementations so it's
obvious what is and isn't worth doing. I was sort of surprised that
auto-vectorization didn't do better, and was pleased to learn that
there's no reason at all to write Neon intrinsics.
I'll delete hello-neon after this merges and I've fixed up the doc
links.
https://github.com/android/ndk-samples/issues/1011
