sbc: ARM NEON optimized joint stereo processing in SBC encoder

sbc: ARM NEON optimized joint stereo processing in SBC encoder

Improves SBC encoding performance when joint stereo is used, which
is a typical A2DP configuration.

Benchmarked on ARM Cortex-A8:

== Before: ==

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

real    0m5.239s
user    0m4.805s
sys     0m0.430s

samples  %        image name               symbol name
26083    25.0856  sbcenc                   sbc_pack_frame
21548    20.7240  sbcenc                   sbc_calc_scalefactors_j
19910    19.1486  sbcenc                   sbc_analyze_4b_8s_neon
14377    13.8272  sbcenc                   sbc_calculate_bits
9990      9.6080  sbcenc                   sbc_enc_process_input_8s_be
8667      8.3356  no-vmlinux               /no-vmlinux
2263      2.1765  sbcenc                   sbc_encode
696       0.6694  libc-2.10.1.so           memcpy

== After: ==

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

real    0m4.389s
user    0m3.969s
sys     0m0.422s

samples  %        image name               symbol name
26234    29.9625  sbcenc                   sbc_pack_frame
20057    22.9076  sbcenc                   sbc_analyze_4b_8s_neon
14306    16.3393  sbcenc                   sbc_calculate_bits
9866     11.2682  sbcenc                   sbc_enc_process_input_8s_be
8506      9.7149  no-vmlinux               /no-vmlinux
5219      5.9608  sbcenc                   sbc_calc_scalefactors_j_neon
2280      2.6040  sbcenc                   sbc_encode
661       0.7549  libc-2.10.1.so           memcpy