x86/vdso: Mark the TSC clocksource path likely

Jumping out of line for the TSC clcoksource read is creating awful
code. TSC is likely to be the clocksource at least on bare metal and the PV
interfaces are sufficiently more work that the jump over the TSC read is
just in the noise.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Link: https://lkml.kernel.org/r/20200207124402.328922847@linutronix.de