Don't ask for bug reports about pthread_is_threaded_np() != 0.

Don't ask for bug reports about pthread_is_threaded_np() != 0.

We thought that this condition was unreachable in ExitPostmaster,
but actually it's possible if you have both a misconfigured locale
setting and some other mistake that causes PostmasterMain to bail
out before reaching its own check of pthread_is_threaded_np().

Given the lack of other reports, let's not ask for bug reports if
this occurs; instead just give the same hint as in PostmasterMain.

Bug: #18783
Reported-by: anani191181515@gmail.com
Author: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Noah Misch <noah@leadboat.com>
Discussion: https://postgr.es/m/18783-d1873b95a59b9103@postgresql.org
Discussion: https://postgr.es/m/206317.1737656533@sss.pgh.pa.us
Backpatch-through: 13