Because new glibc has changed `MINSIGSTKSZ` to be a syscall instead
of being constant, the signal posix handling needed changes, as it
used the value in constexpr context, for deciding size of an array.
It would be simple to fix it by having the handler determine the
signal handling stack size and allocate the memory every time the
handler is being installed, but that would add another allocation
and a syscall every time a test case is entered.

Instead, I split apart the idea of preparing fatal error handlers,
and engaging them, so that the memory can be allocated only once
and still be guarded by RAII.

Also turns out that Catch2's use of `MINSIGSTKSZ` was wrong, and
we should've been using `SIGSTKSZ` the whole time, which we use now.

Closes #2178

Seen with PGI compiler for `static constexpr`:
```
warning: storage class is not first
```

Until now, the stack size for POSIX signal handling was determined by
the implementation defined limit `STKSZ`, which in some cases turned out
to be insufficient, leading to stack overflow inside the signal handler.
The new size, which was determined experimentally, is the larger of 32kb
or `MINSTKSZ`.

Fixes #1225

The warning occurred when !CATCH_CONFIG_WINDOWS_SEH
&& !CATCH_CONFIG_POSIX_SIGNALS.

It was a bit of a mess previously