Bug 1842933 - Use NEON_FLAGS instead of VPX_ASFLAGS for libaom neon code. r=firefox-build-system-reviewers,andi

Back when libaom was added, VPX_ASFLAGS was more or less the only
variable that had the right set of flags for NEON support, which
justified its use. However, it's not set when building against system
libvpx.

Bug 1791482 improved things, though, and VPX_ASFLAGS, as far as NEON is
concerned, is now only re-exporting NEON_FLAGS, with the same caveat that
VPX_ASFLAGS is empty when building against system libvpx.

So we should use NEON_FLAGS instead of VPX_ASFLAGS.

Differential Revision: https://phabricator.services.mozilla.com/D183310

media/libaom/moz.build

@@ -55,7 +55,7 @@ elif CONFIG['CPU_ARCH'] == 'arm':
 
     for f in SOURCES:
         if f.endswith('neon.c'):
-            SOURCES[f].flags += CONFIG['VPX_ASFLAGS']
+            SOURCES[f].flags += CONFIG['NEON_FLAGS']
 
     if CONFIG['OS_TARGET'] == 'Android':
         # For cpu-features.h