ARM: Make arch_specific_insn a define for new arch_probes_insn structure

Because the common underlying code for ARM kprobes and uprobes needs
to share a common architecrure-specific context structure, and because
the generic kprobes include file insists on defining this to a dummy
structure when kprobes is not configured, a new common structure is
required which can exist when uprobes is configured without kprobes.
In this case kprobes will define a dummy structure, but without the
define aliasing the two structure tags it will not affect uprobes and
the shared probes code.

Signed-off-by: David A. Long <dave.long@linaro.org>
Acked-by: Jon Medhurst <tixy@linaro.org>