turns out gcc generates such stackprotector-failure sequences
in certain circumstances:

        movq    -8(%rbp), %rax  # D.16032,
        xorq    %gs:40, %rax    #,
        jne     .L17    #,
        leave
        ret
.L17:
        call    __stack_chk_fail        #
        .size   __stack_chk_test_func, .-__stack_chk_test_func
        .section        .init.text,"ax",@progbits
        .type   panic_setup, @function
panic_setup:
        pushq   %rbp    #

note that there's no jump back to the failing context after the
call to __stack_chk_fail - i.e. it has a ((noreturn)) attribute.

Which is fair enough in the normal case but kills the self-test.
(as we cannot reliably return in the self-test)
Signed-off-by: NIngo Molnar <mingo@elte.hu>