x86/float_dsp: unroll loop in vector_fmac_scalar

~6% faster SSE2 performance. AVX/FMA3 are unaffected.

Signed-off-by: James Almer <jamrial@gmail.com>
Reviewed-by: Christophe Gisquet <christophe.gisquet@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>