arm64: msac: Implement NEON msac_decode_symbol_adapt

                             Cortex A53    A72    A73
msac_decode_symbol_adapt4_c:      107.5   57.1   67.8
msac_decode_symbol_adapt4_neon:    70.1   53.4   55.0
msac_decode_symbol_adapt8_c:      157.3   74.5   90.2
msac_decode_symbol_adapt8_neon:    75.3   57.9   56.2
msac_decode_symbol_adapt16_c:     257.4  106.3  136.0
msac_decode_symbol_adapt16_neon:  101.2   61.2   65.8

Total decoding speedup of Chimera is around 0.8%.

@janne Do you have an opinion on the use of macros here? I'm avoiding duplicating the main ~60 line block of SIMD code by templating it out to three versions. Templating between widths 4 and 8 is trivial (just changing between .4h and .8h register specifiers), but templating between using one or two registers (for width 8 vs 16) is done with a lot of small macros, one per instruction type. The macro definitions end up using more lines of code than it would be to duplicate the code once more...