Suppress "has no symbols" linker warnings on macOS.

After a recent macOS update, building Postgres produces warnings
that look like this:

    ranlib: warning: 'libpgport_shlib.a(pg_cpu_x86.c.o)' has no symbols
    ranlib: warning: 'libpgport_shlib.a(pg_popcount_x86.c.o)' has no symbols

To fix, add a dummy symbol to files that may otherwise have none.
Per project policy, this is a candidate for back-patching into
out-of-support branches: it suppresses annoying compiler warnings
but changes no behavior.

Reported-by: Zhang Mingli <zmlpostgres@gmail.com>
Reviewed-by: John Naylor <johncnaylorls@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/229aaaf3-f529-44ed-8e50-00cb6909af21%40Spark
Backpatch-through: 13

src/port/pg_popcount_aarch64.c

@@ -478,4 +478,10 @@ pg_popcount_masked_neon(const char *buf, int bytes, bits8 mask)
 	return popcnt;
 }
 
-#endif							/* POPCNT_AARCH64 */
+#else							/* POPCNT_AARCH64 */
+
+/* prevent linker complaints about empty module */
+extern int	pg_popcount_aarch64_dummy_variable;
+int			pg_popcount_aarch64_dummy_variable = 0;
+
+#endif							/* ! POPCNT_AARCH64 */

src/port/pg_popcount_avx512.c

@@ -220,4 +220,11 @@ pg_popcount_masked_avx512(const char *buf, int bytes, bits8 mask)
 }
 
 #endif							/* TRY_POPCNT_X86_64 */
-#endif							/* USE_AVX512_POPCNT_WITH_RUNTIME_CHECK */
+
+#else							/* USE_AVX512_POPCNT_WITH_RUNTIME_CHECK */
+
+/* prevent linker complaints about empty module */
+extern int	pg_popcount_avx512_dummy_variable;
+int			pg_popcount_avx512_dummy_variable = 0;
+
+#endif							/* ! USE_AVX512_POPCNT_WITH_RUNTIME_CHECK */