x86, arm: Allow fpu_kern_enter/leave while cold.

Normally these are forbidden above IPL_VM, so that FPU usage doesn't
block IPL_SCHED or IPL_HIGH interrupts.  But while cold, e.g. during
builtin module initialization at boot, all interrupts are blocked
anyway so it's a moot point.

Also initialize x86 cpu_info_primary.ci_kfpu_spl to -1 so we don't
trip over an assertion about it while cold -- the assertion is meant
to detect reentrance into fpu_kern_enter/leave, which is prohibited.

Also initialize cpu0's ci_kfpu_spl.

(riastradh)