x86: put get_kernel_rpl in a common location

This macro is useful for both i386 and x86_64, so put it in a common
location, where both arches can grab it.

Signed-off-by: Glauber de Oliveira Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>