x86: synth filter float: implement SSE2 version

Timings for Arrandale:
          C    SSE
win32:  2108   334
win64:  1152   322

Factorizing the inner loop with a call/jmp is a >15 cycles cost, even with
the jmp destination being aligned.

Unrolling for ARCH_X86_64 is a 20 cycles gain.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/synth_filter.c

@@ -61,4 +61,5 @@ av_cold void ff_synth_filter_init(SynthFilterContext *c)
     c->synth_filter_float = synth_filter_float;
 
     if (ARCH_ARM) ff_synth_filter_init_arm(c);
+    if (ARCH_X86) ff_synth_filter_init_x86(c);
 }