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
Swept:
`-Wpadded` in some places (where it caused extra size, instead of just
saying "hey, we padded struct at the end to align, just as standard says")
`-Wweak-vtables` everywhere (Clang)
`-Wexit-time-destructors` everywhere (Clang)
`-Wmissing-noreturn` everywhere (Clang)
The last three are enabled for Clang compilation going forward.
Also enabled `-Wunreachable-code` for Clang and GCC
