fiber: use alternative signal stack

We install a signal handler that prints the stack trace on SIGSEGV,
SIGBUS, SIGILL, SIGFPE. The signal handler uses the current stack.
This works fine for most issues, but not for stack overflow, because
the latter makes the current stack unusable, leading to a crash in
the signal handler. Let's install an alternative signal stack in each
thread so that we can print the stack trace on stack overflow.

Note that we skip this for ASAN because it installs its own signal
stack. (Installing a custom stack would result in a crash.)

Closes #9222

NO_DOC=bug fix