[ARM] call undefined instruction exception handler with irqs enabled

Aaro says:
> With spinlock debugs enabled I get might_sleep() warnings when using
> ptrace.

tracked down to a missing enable_irq before calling do_undefinstr().

Reported-by: Aaro Koskinen <aaro.koskinen@nokia.com>
Tested-by: Aaro Koskinen <aaro.koskinen@nokia.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>