ARM: kprobes: Add BLX macro

This is for use by inline assembler which will be added to kprobes-arm.c
It saves memory when used on newer ARM architectures and also provides
correct interworking should ARM probes be required on Thumb kernels in
the future.

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