make Linux bootable on ARM again

Commit 200b812d "Clear the exclusive monitor when returning from an
exception" broke the vast majority of ARM systems in the wild which are
still pre ARMv6.  The kernel is crashing on the first occurrence of an
exception due to the removal of the actual return instruction for them.
Let's add it back.

Signed-off-by: Nicolas Pitre <nico@marvell.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>