Optimize popcount functions with ARM Neon intrinsics.

Optimize popcount functions with ARM Neon intrinsics.

This commit introduces Neon implementations of pg_popcount{32,64},
pg_popcount(), and pg_popcount_masked().  As in simd.h, we assume
that all available AArch64 hardware supports Neon, so we don't need
any new configure-time or runtime checks.  Some compilers already
emit Neon instructions for these functions, but our hand-rolled
implementations for pg_popcount() and pg_popcount_masked()
performed better in testing, likely due to better instruction-level
parallelism.

Author: "Chiranmoy.Bhattacharya@fujitsu.com" <Chiranmoy.Bhattacharya@fujitsu.com>
Reviewed-by: John Naylor <johncnaylorls@gmail.com>
Discussion: https://postgr.es/m/010101936e4aaa70-b474ab9e-b9ce-474d-a3ba-a3dc223d295c-000000%40us-west-2.amazonses.com