[ARM] 3746/2: Userspace helpers must be Thumb mode interworkable

Patch from Nicolas Pitre

The userspace helpers in clean/arch/arm/kernel/entry-armv.S are called
directly in/from userspace. They need to cope with being called from
Thumb code.

Patch below uses the bx interworking instruction when
CONFIG_ARM_THUMB=y.

Based on an earlier patch from Paul Brook <paul@codesourcery.com>

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>