Use run-time feature detection for Neon DotProd variance

Use run-time feature detection for Neon DotProd variance

Arm Neon DotProd implementations of vpx_variance<w>x<h> currently
need to be enabled at compile time since they're guarded by #ifdef
feature macros. Now that run-time feature detection has been enabled
for Arm platforms, expose these implementations with distinct
*neon_dotprod names in a separate file and wire them up to the build
system and rtcd.pl. Also add new test cases for the new functions.

Remove the _neon suffix in functions making reference to
vpx_variance<w>x<h>_neon() (e.g. sub-pixel variance) - enabling use
of the appropriate *neon or *neon_dotprod version at run time.

Similar changes for the specialty variance and MSE functions will be
made in a subsequent commit.

Change-Id: I69a0ef0d622ecb2d15bd90b4ace53273a32ed22d