This only showed up on two known platforms, both ia64 (one HP-UX and one Linux wich icc).
For some reason, they only get half the stack size they are supposed to have, which
makes deep SP recursion overrun the stack before check_stack_overrun() is triggered.
Also made som minor fixes in the check_stack_overrun() definition, supporting variable
margins.
No test case added, but the reason for the bug report was a failed existing test case
on these machines, which now works.