kernel: Add interface for hardware shadow stack usage

In order to allow kernel created threads (such as main and idle threads)
to make use of hardware shadow stack implementation, add an interface
for them.

This patch basically provides an infra that architectures need to
implement to provide hardware shadow stack.

Also, main and idle threads are updated to make use of this interface
(if hardware shadow stacks are enabled).

Signed-off-by: Ederson de Souza <ederson.desouza@intel.com>