ARM: 8064/1: fix v7-M signal return

According to the ARM ARM, the behaviour is UNPREDICTABLE if the PC read
from the exception return stack is not half word aligned.  See the
pseudo code for ExceptionReturn() and PopStack().

The signal handler's address has the bit 0 set, and setup_return()
directly writes this to regs->ARM_pc.  Current hardware happens to
discard this bit, but QEMU's emulation doesn't and this makes processes
crash.  Mask out bit 0 before the exception return in order to get
predictable behaviour.

Fixes: 19c4d593

 ("ARM: ARMv7-M: Add support for exception handling")

Cc: stable@kernel.org
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Rabin Vincent <rabin@rab.in>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>