ARM: kprobes: Fix emulation of CMP, CMN, TST and TEQ instructions.

Probing these instructions was corrupting R0 because the emulation code
didn't account for the fact that they don't write a result to a
register.

Signed-off-by: Jon Medhurst <tixy@yxit.co.uk>
Signed-off-by: Nicolas Pitre <nicolas.pitre@linaro.org>