arm64: port synth_filter_float_neon from arm

~25% faster dts decoding overall. The checkasm CPU cycles numbers are
not that useful since synth_filter_float() calls FFTContext.imdct_half().

                         cortex-a57   cortex-a53
synth_filter_float_c:    1866.2       3490.9
synth_filter_float_neon:  915.0       1531.5

With fftc.imdct_half forced to imdct_half_neon:
                         cortex-a57   cortex-a53
synth_filter_float_c:    1718.4       3025.3
synth_filter_float_neon:  926.2       1530.1
