Disable NEON code on Native Client

The NEON assember in libwebp has not yet been ported
to Native Client. This changes disables it.
Related issue:
https://code.google.com/p/nativeclient/issues/detail?id=3205

(cherry picked from commit ac4f5784a0a5d64d54f03dcf2e2d133c4b266faa)

Change-Id: I200291db7aa79d40c1f10cff7622c9b8599e6886
1 file changed