kernel: declare main and idle stack as globals

Renamed main_stack and idle_stack, to _main_stack and
_idle_stack, respectively, and made them globals. This does
not affect performance. They are still kept kernel private
symbols and not part of kernel API.

This will allow these symbols to be referenced in calls to
stack_analyse misc functions to profile stack usage in
applications.

Change-id: Id6b746c5cfda617c26901c6e62c3e17114471f57
Signed-off-by: Vinayak Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>