Pass neon option to compiler, not assembler, so ifdefs will work.

BUG=371
TESTED=locally tested by arm
R=harryjin@google.com

Review URL: https://webrtc-codereview.appspot.com/32879004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1210 16f28f9a-4ce2-e073-06de-1de4eb20be90

libyuv.gyp

@@ -57,7 +57,7 @@
             # arm64 does not need -mfpu=neon option as neon is not optional
             ['target_arch != "arm64"', {
               'cflags': [
-                '-Wa,-mfpu=neon',
+                '-mfpu=neon',
               ],
             }],
           ],