x86/local_apic.c: Add support for installing a thermal interrupt handler

The thermal interrupt is initially masked.

Thermal interrupt handling is enabled by calling lapic_enable_thermal(),
which installs a (single) handler.

[olce: Wrote the commit message.]

Reviewed by: kib, olce
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D44454