aarch64: NEON float FFT

Approximately as fast as the ARM NEON version on Apple's A7.
aarch64: NEON float (i)MDCT

Approximately as fast as the ARM NEON version on Apple's A7.
aarch64: NEON float to s16 audio conversion
aarch64: NEON fixed/floating point MPADSP apply_window

30%/25% (fixed/float) faster mp3 decoding on Apple's A7. The floating
point decoder is approximately 7% faster.
aarch64: NEON vorbis_inverse_coupling

From the ARMv7 NEON version. 16 times faster as the C version, overall
more than 12% faster vorbis decoding on Apple's A7.