implement __kernel attribute

[zephyrbot]

Reported by Andrew Boie:

For systems that enable CONFIG_APPLICATION_MEMORY, we need a way for application code to reserve memory which will end up in kernel space. This is needed for apps to define toplevel kernel objects.

example:

struct k_sem __kernel my_semaphores[16];

These semaphore objects will all end up in kernel and not application data section.

(Imported from Jira ZEP-2366)