ARM: kprobes: Add common decoding function for LDM and STM

The encoding of these instructions is substantially the same for both
ARM and Thumb, so we can have common decoding and simulation functions.

This patch moves the simulation functions from kprobes-arm.c to
kprobes-common.c. It also adds a new simulation function
(simulate_ldm1_pc) for the case where we load into PC because this may
need to interwork.

The instruction decoding is done by a custom function
(kprobe_decode_ldmstm) rather than just relying on decoding table
entries because we will later be adding optimisation code.

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