pgsql: Move x86-64-specific popcount code to pg_popcount_x86.c.

Move x86-64-specific popcount code to pg_popcount_x86.c.

This moves the remaining x86-64-specific popcount implementations
in pg_bitutils.c to pg_popcount_x86.c.

Reviewed-by: John Naylor <johncnaylorls@gmail.com>
Discussion: https://postgr.es/m/aWf_InS1VrbeXAfP%40nathan

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/79e232ca013c7f357704f8af9782fe72466c216e

Modified Files
--------------
src/include/port/pg_bitutils.h |  20 ++--
src/port/pg_bitutils.c         | 263 +---------------------------------------
src/port/pg_popcount_x86.c     | 264 +++++++++++++++++++++++++++++++++++++++--
3 files changed, 267 insertions(+), 280 deletions(-)