Refactor code for setting pg_popcount* function pointers.

Refactor code for setting pg_popcount* function pointers.

Presently, there are three copies of this code, and a proposed
follow-up patch would add more code to each copy.  This commit
introduces a new inline function for this code and makes use of it
in the pg_popcount*_choose functions, thereby reducing code
duplication.

Author: Paul Amonson
Discussion: https://postgr.es/m/BL1PR11MB5304097DF7EA81D04C33F3D1DCA6A%40BL1PR11MB5304.namprd11.prod.outlook.com