x86: provide paravirtualized hook for rdtscp

This patch adds a field in pv_cpu_ops for a paravirtualized hook
for rdtscp, needed for x86_64.

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>