x86: standardize Kbuild rules

Introducing this Kbuild file allow us to:

    make arch/x86/

And thus building all the core part of x86.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Jaswinder Singh Rajput <jaswinder@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>