Initialize the count of saved register windows to 0 in the pcb created

for the new thread.  The rest of the fields in the pcb wind up being
written to before they're read as a normal part of the pcb usage but
this field may be read upon return to userland, having it be uninitialized
garbage is bad.

Submitted by:	Andrew Belashov (bel at orel dot ru)
Reviewed by:	jake
MFC after:	3 days

sys/sparc64/sparc64/vm_machdep.c

@@ -141,6 +141,7 @@ cpu_thread_setup(struct thread *td)
 
 	pcb = (struct pcb *)((td->td_kstack + td->td_kstack_pages * PAGE_SIZE -
 	    sizeof(struct pcb)) & ~0x3fUL);
+	pcb->pcb_nsaved = 0;
 	td->td_frame = (struct trapframe *)pcb - 1;
 	td->td_pcb = pcb;
 }