kernel: prepare threads after PRE_KERNEL*

prepare_multithreading() was done very early as it had a call
to initialize the interrupt subsystem. This was causing problems
with stack pointer randomization as any HW-based entropy drivers
had not been initialized.

Move the call to initialize the interrupt system out of
prepare_multithreading(), which now really does just prepare
the system to start threads. This is now done after the PRE_KERNEL
phases.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>