RIght now PCB_KERNFPU is used both as indication that kernel prepared hardware FPU context to use and that the thread is fpu-kern thread. This also breaks fpu_kern_enter(FPU_KERN_NOCTX), since fpu_kern_leave() then clears PCB_KERNFPU.
Introduce new flag PCB_KERNFPU_THR which indicates that the thread is fpu-kern. Do not clear PCB_KERNFPU if fpu-kern thread leaves noctx fpu region.
Reported by: jhb