ARM: kprobes-test: use <asm/opcodes.h> for instruction accesses

Ensure we read instructions in the correct endian-ness by using
the <asm/opcodes.h> helper to transform them as necessary.

Acked-by: Jon Medhurst <tixy@linaro.org>
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
[taras.kondratiuk@linaro.org: fix next_instruction() function]
Signed-off-by: Taras Kondratiuk <taras.kondratiuk@linaro.org>