pgsql: Suppress "has no symbols" linker warnings on macOS.

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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/3dd42ee97b812c0711b4d18e9cab069065367dca

Modified Files
--------------
src/port/pg_cpu_x86.c | 7 ++++++-
src/port/pg_popcount_aarch64.c | 8 +++++++-
src/port/pg_popcount_x86.c | 8 +++++++-
3 files changed, 20 insertions(+), 3 deletions(-)