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.
dcadsp: split synth_filter_float

The goal is to avoid function calls within assembly which are a mess on x86.
dca: replace some memcpy by AV_COPY128
fmtconvert: int32_t input to int32_to_float_fmul_scalar

It was previously declared as int.
Does not change fate results for x86.
dca: factorize scaling in inverse ADPCM

The codeblock affected accounted for around 4% of the runtime on x86_64
(measured using oprofile on a Penryn).
Timings for Arrandale (gcc 4.6.1 tdm64-1 for windows):
win32: 341 -> 331
win64: 321 -> 120
Part of the gain also comes from the adpcm values to be converted to float
outside of the loops.