sbc: ARM NEON optimization for scale factors calculation

sbc: ARM NEON optimization for scale factors calculation

Improves SBC encoding performance when joint stereo is not used.
Benchmarked on ARM Cortex-A8:

== Before: ==

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

real    0m4.756s
user    0m4.313s
sys     0m0.438s

samples  %        image name               symbol name
2569     27.6296  sbcenc                   sbc_pack_frame
1934     20.8002  sbcenc                   sbc_analyze_4b_8s_neon
1386     14.9064  sbcenc                   sbc_calculate_bits
1221     13.1319  sbcenc                   sbc_calc_scalefactors
996      10.7120  sbcenc                   sbc_enc_process_input_8s_be
878       9.4429  no-vmlinux               /no-vmlinux
204       2.1940  sbcenc                   sbc_encode
56        0.6023  libc-2.10.1.so           memcpy

== After: ==

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

real    0m4.220s
user    0m3.797s
sys     0m0.422s

samples  %        image name               symbol name
2563     31.3249  sbcenc                   sbc_pack_frame
1892     23.1239  sbcenc                   sbc_analyze_4b_8s_neon
1368     16.7196  sbcenc                   sbc_calculate_bits
961      11.7453  sbcenc                   sbc_enc_process_input_8s_be
836      10.2176  no-vmlinux               /no-vmlinux
262       3.2022  sbcenc                   sbc_calc_scalefactors_neon
199       2.4322  sbcenc                   sbc_encode
49        0.5989  libc-2.10.1.so           memcpy