x86/swr: add ff_float_to_int32_a_avx2

13797 decicycles in ff_float_to_int32_a_sse2, 32768 runs, 0 skips
8603 decicycles in ff_float_to_int32_a_avx2, 32766 runs, 2 skips

Reviewed-by: Christophe Gisquet <christophe.gisquet@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>