dsp/WEBP_USE_NEON: test for __aarch64__

__ARM_NEON__ is unset by current linux gcc/clang + android toolchains
for aarch64/arm64 builds.

Change-Id: Ib2ca172ea6fcf046e4ced19a431088674c99b7f6
Android.mk: always include *_neon.c in the build

the inclusion of the files is harmless when NEON is not enabled and will
allow them to be built with NEON for APP_ABI=arm64-v8a which currently
does not use the '.neon' suffix

Change-Id: I39377876b1b68822c38f4e2396da93c56145fc0f
Merge "dsp/WEBP_USE_NEON: test for __aarch64__"