Bug#757785: libwebp FTBFS on arm64, internal compiler errors

[Jeff Breidenbach]

Upstream says:

   GCC 4.9 strikes again. CFLAGS='-O2 -frename-registers' should do [the
trick].

I'm generally comfortable with NMU high urgency situations
like this one. I prefer to keep the optimizations if possible,
because they are substantial.

  * NEON assembly additions:
    - ~25% faster lossy decode / encode (-m 4)
    - ~10% faster lossless decode
    - ~5-10% faster lossless encode (-m 3/4)