x86/apic: Fix UP boot crash

Commit 31b3c9d72340 ("xen/x86: Implement x86_apic_ops") implemented
this:

... without considering that on UP the function pointer might be NULL.

Cc: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Link: http://lkml.kernel.org/n/tip-3pfty0ml4yp62phbkchichh0@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>