Enable 32-bit armv8 NEON for libmpeg2

Since ARM neon instructions were only used on armv7-a-neon
architecture variant, the default implementation for this
library doesn't use these advanced SIMD instructions.

Bug: 65568191
Test: lunch aosp_arm64; emulator # on oc-mr1-dev; boot to home screen
      Check NEON related objects are included in the libmpeg2dec.a
Change-Id: Ic807a949e154cf1c184ba84a2b751e439c1dfca6