arch/{arm, x86}: Add cpu_idle function

Signed-off-by: Pawel Dziepak <pdziepak@quarnos.org>
arch/x86, kernel: Add basic stubs of thread implementation

Signed-off-by: Pawel Dziepak <pdziepak@quarnos.org>
arch/arm: Add basic context switch routines

Signed-off-by: Pawel Dziepak <pdziepak@quarnos.org>