ARM: added 'neon_composite_over_n_8888_8888_ca' fast path

ARM: added 'neon_composite_over_n_8888_8888_ca' fast path

This fast path function improves performance of 'firefox-talos-gfx'
cairo-perf trace.

Benchmark from ARM Cortex-A8 @720MHz

before:

[ # ]  backend                         test   min(s) median(s) stddev. count
[  0]    image            firefox-talos-gfx  139.969  141.176   0.35%    6/6

after:

[ # ]  backend                         test   min(s) median(s) stddev. count
[  0]    image            firefox-talos-gfx  111.810  112.196   0.23%    6/6