context: Increase a gccgo-specific alloc limit.

Change-Id: I526ecbab37b68d689ddffa60c2e231099c1477b6
Reviewed-on: https://go-review.googlesource.com/13056
Reviewed-by: Dave Day <djd@golang.org>

context/context_test.go

@@ -375,7 +375,7 @@ func TestAllocs(t *testing.T) {
 				<-c.Done()
 			},
 			limit:      8,
-			gccgoLimit: 13,
+			gccgoLimit: 15,
 		},
 		{
 			desc: "WithCancel(bg)",