Previously, we could run out of stack handling stack overflows, leaving
users with unspecific SIGSEGV crashes and no panic message.

By providing a single alternative stack exclusively for SIGSEGV handling
where sigaltstack() is available, we increase chances for our signal handler
to finish successfully.

In particular, this will make it easier to diagnose stack overflows by
comparing the failing address with the stack info from the panic output.

This could be further improved by giving advise to increase thread_pool_stack
if si_addr is near the stack boundaries.

c00057.vtc now triggers a stack overflow instead of raising a SIGSEGV.

Merges #2396