Load the new sp_el0 with interrupts disabled in fork_trampoline. If an
interrupt arrives in fork_trampoline after sp_el0 was written we may then
switch to a new thread, enter userland so change this stack pointer, then
return to this code with the wrong value. This fixes this case by moving
the load of sp_el0 until after interrupts have been disabled.

Reported by: Mark Millard (markmi@dsl-only.net)
Sponsored by: ABT Systems Ltd
Differential Revision: https://reviews.freebsd.org/D9593