Use run-time CPU feature detection for Neon DotProd SAD

Use run-time CPU feature detection for Neon DotProd SAD

Arm Neon DotProd implementations of vpx_sad* 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 separate files and wire them up to the build system and
rtcd.pl. Also add new test cases for the new DotProd functions.

Change-Id: Ic6906c28240276ba89787eadbc9393a232374f95