x86: split get_cycles_sync

This patch splits get_cycles_sync() into  __get_cycles_sync(),
and the rdtscll part. Paravirt guests cannot issue rdtscl directly,
as it involves a function call in vdso area.

So, using the __get_cycles_sync() base, we introduce vget_cycles_sync,
which then calls the native version of rdtscll. Ideally, however, a guest
should define its own clocksource, together with a vread function

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>

arch/x86/kernel/tsc_64.c

@@ -306,7 +306,7 @@ static cycle_t read_tsc(void)
 
 static cycle_t __vsyscall_fn vread_tsc(void)
 {
-	cycle_t ret = (cycle_t)get_cycles_sync();
+	cycle_t ret = (cycle_t)vget_cycles_sync();
 	return ret;
 }