i386: supply thermal interrupt handler

This fixes a build break for i386.

Reviewed by: kib, olce, Koine Yuusuke <koinec@yahoo.co.jp>
Fixes: 87ba088fa310 ("x86/local_apic.c: Add support for installing a thermal interrupt handler")
Differential Revision: https://reviews.freebsd.org/D58332

(cherry picked from commit cb325dcedfa291c9bfe350a513694df3776a17a4)