arch: raise an error when compiling for an unknown ARM platform

The nop, read_barrier() and write_barrier() macros have to always be
defined but due to the way os/arch-arm.h is written it is possible for
them to be undefined when the ARM platform is not recognized.

Change things so compiling for an unknown ARM platform raises an
explicit compilation error at the point of the problem.

Signed-off-by: Sitsofe Wheeler <sitsofe@yahoo.com>