sbc: MMX optimization for scale factors calculation

sbc: MMX optimization for scale factors calculation

Improves SBC encoding performance when joint stereo is not used.
Benchmarked on Pentium-M:

== Before: ==

$ time ./sbcenc -b53 -s8 test.au > /dev/null

real    0m1.439s
user    0m1.336s
sys     0m0.104s

samples  %        image name               symbol name
8642     33.7473  sbcenc                   sbc_pack_frame
5873     22.9342  sbcenc                   sbc_analyze_4b_8s_mmx
4435     17.3188  sbcenc                   sbc_calc_scalefactors
4285     16.7331  sbcenc                   sbc_calculate_bits
1942      7.5836  sbcenc                   sbc_enc_process_input_8s_be
322       1.2574  sbcenc                   sbc_encode

== After: ==

$ time ./sbcenc -b53 -s8 test.au > /dev/null

real    0m1.319s
user    0m1.220s
sys     0m0.084s

samples  %        image name               symbol name
8706     37.9959  sbcenc                   sbc_pack_frame
5740     25.0513  sbcenc                   sbc_analyze_4b_8s_mmx
4307     18.7972  sbcenc                   sbc_calculate_bits
1937      8.4537  sbcenc                   sbc_enc_process_input_8s_be
1801      7.8602  sbcenc                   sbc_calc_scalefactors_mmx
307       1.3399  sbcenc                   sbc_encode