runtime: eliminate remaining recordspan write barriers

recordspan has two remaining write barriers from writing to the
pointer to the backing store of h.allspans. However, h.allspans is
always backed by off-heap memory, so let the compiler know this.
Unfortunately, this isn't quite as clean as most go:notinheap uses
because we can't directly name the backing store of a slice, but we
can get it done with some judicious casting.

For #22460.

Change-Id: I296f92fa41cf2cb6ae572b35749af23967533877
Reviewed-on: https://go-review.googlesource.com/73414
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/slice.go

@@ -14,6 +14,13 @@ type slice struct {
 	cap   int
 }
 
+// An notInHeapSlice is a slice backed by go:notinheap memory.
+type notInHeapSlice struct {
+	array *notInHeap
+	len   int
+	cap   int
+}
+
 // maxElems is a lookup table containing the maximum capacity for a slice.
 // The index is the size of the slice element.
 var maxElems = [...]uintptr{