enable NEON I8MM optimizations

Apex-Size-Increase: libcodec2_soft_av1enc.so: ~32KiB
Previous-Platform-Support: yes
Aosp-First: true
Test-Info: new instructions will be covered by existing tests with \
  requisite hardware
Bug: 338678296
Test: presubmit + \
      (shiba) atest CtsMediaV2TestCases -- --module-arg \
        CtsMediaV2TestCases:instrumentation-arg:codec-prefix:=c2.android.av1.encoder \
      mmm external/libaom external/webrtc frameworks/av/media
Change-Id: I9a0e885476a0026cd86bcea2276bf7e85d9e8cee