runtime: fix GC to take goroutines into account

This fix is needed because with the new task-based scheduler, the
current stack pointer may not be on the system stack.

src/runtime/gc_stack_raw.go

@@ -9,5 +9,6 @@ package runtime
 // the linker) and getting the current stack pointer from a register. Also, it
 // assumes a descending stack. Thus, it is not very portable.
 func markStack() {
-	markRoots(getCurrentStackPointer(), stackTop)
+	// Mark system stack.
+	markRoots(getSystemStackPointer(), stackTop)
 }