xenpv_exc_nmi() and xenpv_exc_debug() are only defined on 64-bit kernels,
but they snuck into the 32-bit build via <asm/identry.h>, causing the link
to fail:

  ld: arch/x86/entry/entry_32.o: in function `asm_xenpv_exc_nmi':
  (.entry.text+0x817): undefined reference to `xenpv_exc_nmi'

  ld: arch/x86/entry/entry_32.o: in function `asm_xenpv_exc_debug':
  (.entry.text+0x827): undefined reference to `xenpv_exc_debug'

Only use them on 64-bit kernels.

Fixes: f41f0824: ("x86/entry/xen: Route #DB correctly on Xen PV")
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>