ARM: Add 'neon_composite_over_n_8888_0565_ca' fast path

ARM: Add 'neon_composite_over_n_8888_0565_ca' fast path

This improves the performance of the firefox-talos-gfx benchmark with
the image16 backend. Benchmark on an 800 MHz ARM Cortex A8:

Before:

[ # ]  backend                         test   min(s) median(s) stddev. count
[  0]  image16            firefox-talos-gfx  121.773  122.218   0.15%    6/6

After:

[ # ]  backend                         test   min(s) median(s) stddev. count
[  0]  image16            firefox-talos-gfx   85.247   85.563   0.22%    6/6

V2: Slightly better instruction scheduling based on comments from Taekyun Kim.
V3: Eliminate all stalls from the inner loop. Also based on comments from Taekyun Kim.