this extends the brk/stack collision protection added to full malloc
in commit 276904c2 to also protect the
__simple_malloc function used in static-linked programs that don't
reference the free function.

it also extends support for using mmap when brk fails, which full
malloc got in commit 54463033, to
__simple_malloc.

since __simple_malloc may expand the heap by arbitrarily large
increments, the stack collision detection is enhanced to detect
interval overlap rather than just proximity of a single address to the
stack. code size is increased a bit, but this is partly offset by the
sharing of code between the two malloc implementations, which due to
linking semantics, both get linked in a program that needs the full
malloc with realloc/free support.