The linux kernel's trap tables vector all unassigned trap
numbers to BAD_TRAP, which then raises SIGILL.

Tested-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reported-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>