x86: adjust assembly macros on 64-bit as well.

This patch adjust the paravirt macros used in assembly code
to accomodate for x86_64 as well.

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>