hw: sanitize Kernel::Cpu::Context aka Kernel::Cpu_context

[skalk]

The current class Kernel::Cpu_job shall be named Kernel::Cpu::Context consistently (like Kernel::Scheduler::Context). Unfortunately, we cannot define it as inner class of Kernel::Cpu, because of inheritance relations of Thread to Cpu::Context and Cpu::Idle_thread to Thread.
Anyway, at least we can name it Kernel::Cpu_context and provide a Cpu::Context alias.

More important, this class contains a pointer to a Kernel::Cpu object. This needs to be a pointer in case the affinity of the context change. Nonetheless, the pointer is directly useable by its class derivates, and heavily de-referenced without any check. Instead of using a lot of pointer sanity checks (or lambda for it), we should ensure that the Cpu pointer is set correctly at construction time, and always stays valid. Therefore, the Cpu affinity needs to be given with the Thread creation syscall, and not via the Thread start syscall.