Rename pg_popcount_avx512.c to pg_popcount_x86.c.

This is preparatory work for a follow-up commit that will move the
rest of the x86-64-specific popcount code to this file.

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

src/port/Makefile

@@ -47,7 +47,7 @@ OBJS = \
 	pg_localeconv_r.o \
 	pg_numa.o \
 	pg_popcount_aarch64.o \
-	pg_popcount_avx512.o \
+	pg_popcount_x86.o \
 	pg_strong_random.o \
 	pgcheckdir.o \
 	pgmkdirp.o \

src/port/meson.build

@@ -10,7 +10,7 @@ pgport_sources = [
   'pg_localeconv_r.c',
   'pg_numa.c',
   'pg_popcount_aarch64.c',
-  'pg_popcount_avx512.c',
+  'pg_popcount_x86.c',
   'pg_strong_random.c',
   'pgcheckdir.c',
   'pgmkdirp.c',

src/port/pg_popcount_x86.c

@@ -1,12 +1,12 @@
 /*-------------------------------------------------------------------------
  *
- * pg_popcount_avx512.c
- *	  Holds the AVX-512 pg_popcount() implementation.
+ * pg_popcount_x86.c
+ *	  Holds the x86-64 pg_popcount() implementations.
  *
  * Copyright (c) 2024-2026, PostgreSQL Global Development Group
  *
  * IDENTIFICATION
- *	  src/port/pg_popcount_avx512.c
+ *	  src/port/pg_popcount_x86.c
  *
  *-------------------------------------------------------------------------
  */