f00f: We don't need giant to create IDT for workaround.

We don't need to assert we have Giant here. All machines that require
the F00F workaround are UP and interrupts are disabled. Since we are
single threaded, it's safe to allocate the IDT area with pmap_trm_alloc,
interact with the current idt table and replace the IDT table without
any Giant locking.

Sponsored by: Netflix
Reviewed by: kib, markj
Differential Revision: https://reviews.freebsd.org/D31839