previously the neon source code was broken into a separate
library built with -mfpu=neon for the neon assembly, while
the C code was built without neon.

In this change, the neon code is added to the main library
and all code built with neon.

TBR=harryjin@google.com
BUG=libyuv:371

Review URL: https://codereview.chromium.org/1392043003 .