arm64: msac: Avoid 32 bit intermediates in symbol_adapt

This gives small gains on A72 and A73, and on A53 on symbol_adapt16.

Before:                      Cortex A53    A72    A73
msac_decode_symbol_adapt4_neon:    63.2   52.8   53.3
msac_decode_symbol_adapt8_neon:    68.5   57.9   55.7
msac_decode_symbol_adapt16_neon:   92.8   59.7   62.8
After:
msac_decode_symbol_adapt4_neon:    63.3   48.3   50.0
msac_decode_symbol_adapt8_neon:    68.7   55.5   54.0
msac_decode_symbol_adapt16_neon:   88.6   58.8   60.0