NEON intrinsics for encoding

* inverse transform is actually slower with intrinsics + gcc-4.6,
  so is left disabled for now.
  With gcc-4.8, it's a bit faster than inlined assembly.

* Sum of Square error function provide a 2-3% speed up
  There's enabled by default (since there's no inlined-asm equivalent)

Change-Id: I361b3f0497bc935da4cf5b35e330e379e71f498a
lossless_neon: disable VP8LConvert* functions

due to breakage with NDK/gcc-4.6 builds

Change-Id: Id96258e710ee33e08a023354b3227f27da986620