Function _shim_init_signal_stack

#[unsafe(no_mangle)]
pub extern "C-unwind" fn _shim_init_signal_stack()

Allocates and installs a signal stack.

This is to ensure that our signal handlers have enough stack space; otherwise we can run out in managed processes that use small stacks.

This should be called once per thread before any signal handlers run. Panics if already called on the current thread.