cmake: enable -Wshadow partially for in-tree code

This enables -Wshadow to warn about shadow variables on
in tree code under arch/, boards/, drivers/, kernel/,
lib/, soc/, and subsys/.

Note that this does not enable it globally because
out-of-tree modules will probably take some time to fix
(or not at all depending on the project), and it would be
great to avoid introduction of any new shadow variables
in the meantime.

Also note that this tries to be done in a minimally
invasive way so it is easy to revert when we enable
-Wshadow globally. Source files under modules/, samples/
and tests/ are currently excluded because there does not
seem to be a trivial way to add -Wshadow there without
going through all CMakeLists.txt to add the option
(as there are 1000+ files to change).

Signed-off-by: Daniel Leung <daniel.leung@intel.com>