rewrite hello-neon as a generic vectorization sample

[DanAlbert]

This sample probably made more sense back when it was written, but now it's not really guiding people toward doing the right thing.

Much more useful, I think would be rewriting it as a general purpose vectorization demo. Pick some sort of vectorization problem (probably something simple like a matrix multiply), and build an implementation for each of:

• Neon intrinsics
• Clang's arch-generic intrinsics
• SVE
• C++'s <algorithm> stuff, if the problem we pick is suitable for them
• Plain old C++, letting the compiler autovectorize

And then the UI would show the benchmark results of each so people can see the differences.

[DanAlbert]

After this merges and before I delete the old one, https://developer.android.com/ndk/guides/cpu-arm-neon will need to be updated since it points to hello-neon.