sbc: add iwmmxt optimization for sbc for pxa series cpu

sbc: add iwmmxt optimization for sbc for pxa series cpu

Benchmarked on ARM PXA platform:
===  Before (4 bands) ====
$ time  ./sbcenc_orig  -s 4     long.au  > /dev/null
real    0m 2.44s
user    0m 2.39s
sys     0m 0.05s
===  After (4 bands) ====
$ time  ./sbcenc  -s 4     long.au  > /dev/null
real    0m 1.59s
user    0m 1.49s
sys     0m 0.10s

===  Before (8 bands) ====
$ time  ./sbcenc_orig   -s 8     long.au  > /dev/null
real    0m 4.05s
user    0m 3.98s
sys     0m 0.07s
===  After (8 bands) ====
$ time  ./sbcenc  -s 8     long.au  > /dev/null
real    0m 1.48s
user    0m 1.41s
sys     0m 0.06s

===  Before (a2dp usage) ====
$ time  ./sbcenc_orig   -b53 -s8 -j    long.au  > /dev/null
real    0m 4.51s
user    0m 4.41s
sys     0m 0.10s
===  After (a2dp usage) ====
$ time  ./sbcenc   -b53 -s8 -j    long.au  > /dev/null
real    0m 2.05s
user    0m 1.99s
sys     0m 0.06s