Add .PHONY for dist-for-goals targets

Mac builds don't include build/make/core/tasks/general-tests.mk
which causes the general-tests target not to be marked phony.  Have
Soong mark all dist-for-goals targets as phony so it doesn't rely
on other makefiles being present.

Bug: 205928834
Test: forrest
Change-Id: I9394c9c794fc83cde9649aa12a3039526d8206f3

android/makevars.go

@@ -422,6 +422,7 @@ func (s *makeVarsSingleton) writeLate(phonies []phony, dists []dist) []byte {
 	fmt.Fprintln(buf)
 
 	for _, dist := range dists {
+		fmt.Fprintf(buf, ".PHONY: %s\n", strings.Join(dist.goals, " "))
 		fmt.Fprintf(buf, "$(call dist-for-goals,%s,%s)\n",
 			strings.Join(dist.goals, " "), strings.Join(dist.paths, " "))
 	}