Use direct function calls for pg_popcount{32,64} on non-x86 platforms

Use direct function calls for pg_popcount{32,64} on non-x86 platforms

Previously, all pg_popcount{32,64} calls were indirected through
a function pointer, even though we had no fast implementation for
non-x86 platforms. Instead, for those platforms use wrappers around
the pg_popcount{32,64}_slow functions.

Review and additional hacking by David Rowley
Reviewed by Álvaro Herrera

Discussion: https://www.postgresql.org/message-id/flat/CAFBsxsE7otwnfA36Ly44zZO%2Bb7AEWHRFANxR1h1kxveEV%3DghLQ%40mail.gmail.com