armv6: Add ability to generate single-scanline fast paths

These are suitable for use (save for a small shim function to switch
between masked and maskless versions) in the combine_32 array of functions,
which are used for a given operation if no fast path with specific input
and output pixel formats has been found.
arm: Move BIND_COMBINE_U from NEON code to a generic ARM header

This is so that it can be used with armv6 single-scanline functions.
For namespacing reasons, also rename it to PIXMAN_ARM_BIND_COMBINE_U.
armv6: Add OVER combiner

lowlevel-blt-bench results for two example operations, with and without masks,
neither of which has a dedicated fast path at the time of writing:

over_8888_1555

    Before          After
    Mean   StdDev   Mean   StdDev  Confidence  Change
L1  7.1    0.0      11.3   0.0     100.0%      +59.4%
L2  5.7    0.0      9.5    0.1     100.0%      +67.6%
M   5.6    0.0      9.6    0.0     100.0%      +72.4%
HT  4.9    0.0      7.3    0.0     100.0%      +48.8%
VT  4.9    0.0      7.2    0.0     100.0%      +46.7%
R   4.7    0.0      6.9    0.0     100.0%      +46.8%
RT  3.1    0.0      3.9    0.0     100.0%      +24.3%

over_8888_n_1555

    Before          After
    Mean   StdDev   Mean   StdDev  Confidence  Change
L1  4.9    0.0      8.1    0.0     100.0%      +65.1%
L2  4.4    0.0      7.9    0.0     100.0%      +79.5%
M   4.3    0.0      7.9    0.0     100.0%      +84.4%
HT  4.0    0.0      6.3    0.0     100.0%      +56.6%
VT  4.0    0.0      6.2    0.0     100.0%      +54.7%
R   3.9    0.0      6.1    0.0     100.0%      +54.7%
RT  2.7    0.0      3.5    0.0     100.0%      +30.6%