pc64: fix wrong Ureg* argument on note handler (thanks _sl!)

_sl reported crash:

stats 593: suicide: sys: trap: fault write addr=0xffffffff8258d1b0 pc=0x204cc7

; acid 593
/proc/593/text:amd64 plan 9 executable
/sys/lib/acid/port
/sys/lib/acid/amd64
acid: lstk()
notejmp(ret=0x1,j=0x40ac90)+0x13 /sys/src/libc/amd64/notejmp.c:10
alarmed(a=0xffffffff8258d1b0,s=0x7ffffeffea58)+0x3f /sys/src/cmd/stats.c:718
notifier+0x3e /sys/src/libc/port/atnotify.c:15
acid:

note how a in alarmed is a kernel address!

the first Ureg* argument is passed to the note handler in the
RARG (BX) register, which was not loaded when returning to
userspace from syscall() thru forkret(). fix by returning thru
noteret() from syscall().

sys/src/9/pc64/trap.c

@@ -769,6 +769,7 @@ syscall(Ureg* ureg)
 	if(scallnr!=RFORK && (up->procctl || up->nnote)){
 		splhi();
 		notify(ureg);
+		((void**)&ureg)[-1] = (void*)noteret;	/* loads RARG */
 	}
 	/* if we delayed sched because we held a lock, sched now */
 	if(up->delaysched)