Create a gap between the BSS and the stack guard.
Set this gap (STACK_GUARD_MARGIN) to 32 bytes in all variants.
The gap serves two purposes:
- Detect a stack overflow earlier (falsely triggering in overtight
situations is OK!), so that we can hopefully avoid smashing
the heap and have a clean view during the dump.
- Reserve spack space itself for the stack dumping machinery,
which is going to grow the stack even further.
Remove get_stack_guard_test_value() which was unused.
