lkdtm/bugs: Adjust recursion test to avoid elision

While I was able to trick gcc into keeping a pathological recursion,
Clang was not so easily fooled. Instead, switch to using "volatile" and
side-effects to keep the stack variable allocated and to run the function.
Additionally renames "OVERFLOW" to "EXHAUST_STACK" to better describe the
test.

Signed-off-by: Kees Cook <keescook@chromium.org>