Make CELT FFT twiddle complex type aligned

This makes kiss_twiddle_cpx 4-byte aligned (instead of 2-byte) for
fixed-point builds. Tested with an armv6j+nofp development board, CELT
encoding becomes 1.4x as fast, and decoding over 2x.

Performance gain is mostly attributed to the proper alignment of the
static const array mdct_twiddles960.

Co-authored-by: David Gao <davidgao@google.com>
Signed-off-by: Felicia Lim <flim@google.com>