runtime: don't crash if no p in kickoff

The kickoff function for g0 can be invoked without a p, for example
    from mcall(exitsyscall0) in exitsyscall after exitsyscall has cleared
    the p field. The assignment gp.param = nil will invoke a write barrier.
    If gp.param is not already nil, this will require a p. Avoid the problem
    for a specific case that is known to be OK: when the value in gp.param
    is a *g.
    
    Reviewed-on: https://go-review.googlesource.com/46512

From-SVN: r249595

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-29c61dc3c5151df5de9362b7882ccf04679df976
+f107cc8bced1939b0083231fc1ea24669ca4832c
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/runtime/proc.go

@@ -1097,7 +1097,25 @@ func kickoff() {
 	fv := gp.entry
 	param := gp.param
 	gp.entry = nil
+
+	// When running on the g0 stack we can wind up here without a p,
+	// for example from mcall(exitsyscall0) in exitsyscall.
+	// Setting gp.param = nil will call a write barrier, and if
+	// there is no p that write barrier will crash. When called from
+	// mcall the gp.param value will be a *g, which we don't need to
+	// shade since we know it will be kept alive elsewhere. In that
+	// case clear the field using uintptr so that the write barrier
+	// does nothing.
+	if gp.m.p == 0 {
+		if gp == gp.m.g0 && gp.param == unsafe.Pointer(gp.m.curg) {
+			*(*uintptr)(unsafe.Pointer(&gp.param)) = 0
+		} else {
+			throw("no p in kickoff")
+		}
+	}
+
 	gp.param = nil
+
 	fv(param)
 	goexit1()
 }