Enable NEON functions for ARM32

In init functions, whether the code of NEON initialization is compiled
is decided by macros HAS_XXXX_NEON. These macros HAS_XXXX_NEON are defined
according to __ARM_NEON__/LIBYUV_NEON/__aarch64__. For ARM32, "-mfpu=neon"
isn't passed to the files containing init functions. So __ARM_NEON__ is
undefined which leads to NEON functions are disabled.
In this patch, LIBYUV_NEON is added to enable HAS_XXXX_NEON definition for ARM32.

BUG=none
TESTED=libyuv_unittest on android of ARM32/ARM64
R=fbarchard@google.com

Change-Id: Ib4201e7152d2aeb0ba80d18bbbd0487151e4564e

Review URL: https://webrtc-codereview.appspot.com/32959004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1277 16f28f9a-4ce2-e073-06de-1de4eb20be90

libyuv.gyp

@@ -122,6 +122,9 @@
           'dependencies': [
             'libyuv_neon',
           ],
+          'defines': [
+            'LIBYUV_NEON',
+          ],
         }],
         # MemorySanitizer does not support assembly code yet.
         # http://crbug.com/344505