Fix portability fallout from commit dc227eb82.

Fix portability fallout from commit dc227eb82.

Give up on trying to mechanically forbid abort() within libpq.
Even though there are no such calls in the source code, we've now
seen three different scenarios where build toolchains silently
insert such calls: gcc does it for profiling, some platforms
implement assert() using it, and icc does so for no visible reason.
Checking for accidental use of exit() seems considerably more
important than checking for abort(), so we'll settle for doing
that for now.

Also, filter out __cxa_atexit() to avoid a false match.  It seems
that OpenBSD inserts a call to that despite the fact that libpq
contains no C++ code.

Discussion: https://postgr.es/m/3128896.1624742969@sss.pgh.pa.us