lkdtm/bugs: Make double-fault test always available

lkdtm/bugs: Make double-fault test always available

Adjust the DOUBLE_FAULT test to always be available (so test harnesses
don't have to make exceptions more missing tests), and for the
arch-specific tests to "XFAIL" so that test harnesses can reason about
expected vs unexpected failures.

Fixes: 3faf62e5a252 ("lkdtm: Add a DOUBLE_FAULT crash type on x86")
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/202001021226.751D3F869D@keescook
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>