ARMv6: Add fast path for src_x888_0565

Benchmark results, "before" is upstream/master
5f661ee719be25c3aa0eb0d45e0db23a37e76468, and "after" contains this
patch on top.

lowlevel-blt-bench, src_8888_0565, 100 iterations:

       Before          After
      Mean StdDev     Mean StdDev   Confidence   Change
L1    25.9   0.20    115.6   0.70    100.00%    +347.1%
L2    14.4   0.23     52.7   3.48    100.00%    +265.0%
M     14.1   0.01     79.8   0.17    100.00%    +465.9%
HT    10.2   0.03     32.9   0.31    100.00%    +221.2%
VT     9.8   0.03     29.8   0.25    100.00%    +203.4%
R      9.4   0.03     27.8   0.18    100.00%    +194.7%
RT     4.6   0.04     10.9   0.29    100.00%    +135.9%

At most 19 outliers rejected per test per set.

cairo-perf-trace with trimmed traces results were indifferent.

A system-wide perf_3.10 profile on Raspbian shows significant
differences in the X server CPU usage. The following were measured from
a 130x62 char lxterminal running 'dmesg' every 0.5 seconds for roughly
30 seconds. These profiles are libpixman.so symbols only.

Before:

Samples: 63K of event 'cpu-clock', Event count (approx.): 2941348112, DSO: libpixman-1.so.0.33.1
 37.77%  Xorg  [.] fast_fetch_r5g6b5
 14.39%  Xorg  [.] pixman_composite_over_n_8_8888_asm_armv6
  8.51%  Xorg  [.] fast_write_back_r5g6b5
  7.38%  Xorg  [.] pixman_composite_src_8888_8888_asm_armv6
  4.39%  Xorg  [.] pixman_composite_add_8_8_asm_armv6
  3.69%  Xorg  [.] pixman_composite_src_n_8888_asm_armv6
  2.53%  Xorg  [.] _pixman_image_validate
  2.35%  Xorg  [.] pixman_image_composite32

After:

Samples: 31K of event 'cpu-clock', Event count (approx.): 3619782704, DSO: libpixman-1.so.0.33.1
 22.36%  Xorg  [.] pixman_composite_over_n_8_8888_asm_armv6
 13.59%  Xorg  [.] pixman_composite_src_x888_0565_asm_armv6
 12.75%  Xorg  [.] pixman_composite_src_8888_8888_asm_armv6
  6.79%  Xorg  [.] pixman_composite_add_8_8_asm_armv6
  5.95%  Xorg  [.] pixman_composite_src_n_8888_asm_armv6
  4.12%  Xorg  [.] pixman_image_composite32
  3.69%  Xorg  [.] _pixman_image_validate
  3.65%  Xorg  [.] _pixman_bits_image_setup_accessors

Before, fast_fetch_r5g6b5 + fast_write_back_r5g6b5 took 46% of the
samples in libpixman, and probably incurred some memcpy() load, too.
After, pixman_composite_src_x888_0565_asm_armv6 takes 14%. Note, that
the sample counts are very different before/after, as less time is spent
in Pixman and running time is not exactly the same.

Furthermore, in the above test, the CPU idle function was sampled 9%
before, and 15% after.

v4, Pekka Paalanen <pekka.paalanen@collabora.co.uk> :
        Re-benchmarked on Raspberry Pi, commit message.