Fix for mis-interpretation of PCB_KERNFPU.

Fix for mis-interpretation of PCB_KERNFPU.

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 and tested by:	jhb (amd64)
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D25511