Use npxdrop in cpu_thread_exit to save some cycles.

Clear FPU pcb flags for new upcall thread, these flags needn't
be inherited, the new thread should start from clean FPU status.

sys/i386/i386/vm_machdep.c

@@ -286,7 +286,8 @@ cpu_thread_exit(struct thread *td)
 {
 	struct pcb *pcb = td->td_pcb; 
 #ifdef DEV_NPX
-	npxexit(td);
+	if (td == PCPU_GET(fpcurthread))
+		npxdrop();
 #endif
         if (pcb->pcb_flags & PCB_DBREGS) {
 		/* disable all hardware breakpoints */
@@ -369,6 +370,7 @@ cpu_set_upcall(struct thread *td, struct thread *td0)
 	 * more analysis) (need a good safe default).
 	 */
 	bcopy(td0->td_pcb, pcb2, sizeof(*pcb2));
+	pcb2->pcb_flags &= ~(PCB_NPXTRAP|PCB_NPXINITDONE);
 
 	/*
 	 * Create a new fresh stack for the new thread.