Add an SIGSEGV handler to the ContextOutOfBoundsTest to ensure that
the received SIGSEGV is caused by accessing the guard page.

Thanks to Nicolas Pfeiffer for writing the initial prototypical
implementation of continuation stealing and the cactus stack
mechanism, on which this is based.

Co-authored-by: Nicolas Pfeiffer <pfeiffer@cs.fau.de>

The thread local currentContext was already set to nullptr when we
logged it. Use the right field instead.

This function is not performance critical and hence can be moved out
of the header into the compilation unit.

because the hook is also invoked on resumption of the, now unblocked,
context.