gem5: Add gem5 instrumentation

Export a set of symbols used for gem5 to determine sizes and locations
of useful data structures within the kernel.

Change-Id: Idcfaf5fcd91b7ea38e70f174af7f4d19c27a107e
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-by: Geoffrey Blake <Geoffrey.Blake@arm.com>
[Ported to 4.14]
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>