x86: integrate 32-bit and 64-bit code in msr.h

This patches proceeds with the integration of msr.h, making
the code unified, instead of having a version for each architecture.
We stick with the native_* functions, and then paravirt comes for free.

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>