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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/6be53c27673a5fca64a00a684c36c29db6ca33a5

Modified Files
--------------
src/include/port/pg_bitutils.h | 9 ++
src/port/Makefile | 1 +
src/port/meson.build | 1 +
src/port/pg_bitutils.c | 22 +++--
src/port/pg_popcount_aarch64.c | 208 +++++++++++++++++++++++++++++++++++++++++
5 files changed, 235 insertions(+), 6 deletions(-)