runtime: fix saved PC/SP after safe-point function in syscall

Running a safe-point function on syscall entry uses systemstack() and
hence clobbers g.sched.pc and g.sched.sp. Fix this by re-saving them
after the systemstack, just like in the other uses of systemstack in
reentersyscall.

Change-Id: I47868a53eba24d81919fda56ef6bbcf72f1f922e
Reviewed-on: https://go-review.googlesource.com/12125Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/proc1.go

@@ -1831,6 +1831,7 @@ func reentersyscall(pc, sp uintptr) {
 	if _g_.m.p.ptr().runSafePointFn != 0 {
 		// runSafePointFn may stack split if run on this stack
 		systemstack(runSafePointFn)
+		save(pc, sp)
 	}
 
 	_g_.m.syscalltick = _g_.m.p.ptr().syscalltick