x86: prepare consolidation of cpu/ related Makefiles

Prepare the makefiles in x86/kernel/cpu and x86/kernel/cpu/mcheck to
be used by the x86_64 build as well.

No code change.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>