ARM: kprobes: Optimise emulation of LDM and STM

This patch improves the performance of LDM and STM instruction
emulation. This is desirable because.

- jprobes and kretprobes probe the first instruction in a function and,
  when the frame pointer is omitted, this instruction is often a STM
  used to push registers onto the stack.

- The STM and LDM instructions are common in the body and tail of
  functions.

- At the same time as being a common instruction form, they also have
  one of the slowest and most complicated simulation routines.

The approach taken to optimisation is to use emulation rather than
simulation, that is, a modified form of the instruction is run with
an appropriate register context.

Benchmarking on an OMAP3530 shows the optimised emulation is between 2
and 3 times faster than the simulation routines. On a Kirkwood based
device the relative performance was very significantly better than this.

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

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