modules: cmsis: move glue code to modules/cmsis

The CMSIS module glue code was part of arch/ directory. Move it to
modules/cmsis, and provide a single entry point for it: cmsis_core.h.
This entry header will include the right CMSIS header (M or A/R).

To make this change possible, CMSIS module Kconfig/CMake are declared as
external, allowing us to add a new Zephyr include directory.

All files including CMSIS have been updated.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>