dsputil: modify scalarproduct_int16 to handle mod8 numbers of loops.

It was reported that handling such numbers of loops was needed.
ra144: use scalarproduct_int16
dsputil: allow scalarproduct_and_madd_int16 to handle mod8 number of loops

Some callers may pass such number of loops. The x86 SSE2/SSSE3 versions are
modified to have a tail verifying whether a last batch of 8 elements needs
to be processed.
wma lossless: reuse scalarproduct_and_madd_int16

Approximate relative speedup depending on instruction set:
plain C: -6%
mmxext:  51%
sse2:    54%
wmaprolossless: switching back to C dsp function

The DSP function is only guaranteed to handle a number of samples that is a
multiple of 16, but this codec may only guarantees multiples of 8.

However, no such sample is known to date, so rather than not use the
optimized DSP function, whenever such a troublesome case is encountered, the
DSP function pointer is set back to the C one.