Opus encode fails on 32-bit ARM

NEON optimizations overrun buffer due to improper termination condition
Also mark several arm-optimized routines which deliberately cause integer overflow
so they won't trap.

Bug: 136616344
Test: run cts -m CtsMediaTestCases  -t
      android.media.cts.EncoderTest#testOpusEncoders' on 32 bits project

Change-Id: Ib48b17fd1e4b13fe37c0e71d5f33f6f59f1f97d1