x86/hyper-v: Add "polling" bit to hv_synic_sint

That bit is documented in TLFS 5.0c as follows:

  Setting the polling bit will have the effect of unmasking an
  interrupt source, except that an actual interrupt is not generated.

Signed-off-by: Wei Liu <wei.liu@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/20191222233404.1629-1-wei.liu@kernel.org