x86: Fix lapic_ipi_alloc() on i386

The loop which checks to see if "dynamic" IDT entries are allocated
needs to compare with the trampoline address of the reserved ISR.
Otherwise it will never succeed.

Reported by: Harry Schmalzbauer <freebsd@omnilan.de>
Tested by: Harry Schmalzbauer <freebsd@omnilan.de>
Reviewed by: kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D30576