Fix check for trapping exit() calls in libpq

Fix check for trapping exit() calls in libpq

Commit e9bc0441f added an errorhint on the exit() check for libpq, but
accidentally changed the nm commandline to use -a instead of -A. These
options are similar enough to hide it in testing, but -a can also show
debugger symbols which isn't what we want. Fix by reverting the check
back to using -A again.

Reported-by: Anton Voloshin <a.voloshin@postgrespro.ru>
Discussion: https://postgr.es/m/bd2c8409-d6b3-5de9-ba0f-40c1381f630f@postgrespro.ru