swresample/swresample: do not use s32p internally by default when resampling

use fltp when doing s32 -> s32 resampling
because s32p has no simd optimization

benchmark:
old 17.913s
new  7.584s (use fma3)

Reviewed-by: wm4 <nfxjfg@googlemail.com>
Signed-off-by: Muhammad Faiz <mfcc64@gmail.com>