x86: Short-circuit ipi_all_but_self() on UP systems

Apparently this is required on old intel hw, see
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=275086#c3

PR: 275086
Reviewed by: mav, kib
Fixes: 279cd05b7e4d ("Use APIC_IPI_DEST_OTHERS for bitmapped IPIs too.")
MFC after: 1 week
Diagnosed by: Ben Wilber <ben@desync.com>
Differential Revision: https://reviews.freebsd.org/D48361