Convert #NMI to IDTENTRY_NMI:
  - Implement the C entry point with DEFINE_IDTENTRY_NMI
  - Fixup the XEN/PV code
  - Remove the old prototypes

No functional change.
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Reviewed-by: NAlexandre Chartre <alexandre.chartre@oracle.com>
Acked-by: NPeter Zijlstra <peterz@infradead.org>
Acked-by: NAndy Lutomirski <luto@kernel.org>
Link: https://lkml.kernel.org/r/20200505135314.609932306@linutronix.de