Emit LOCAL_CHECK_SAME_VNDK_VARIANTS

If a VNDK library does not require using different core and vendor
variants, emit LOCAL_CHECK_SAME_VNDK_VARIANTS so that the two variants
can be checked for identicalness.

Bug: 145157349
Test: With the corresponding change in build/make, remove libbinder
      from build/soong/cc/config/vndk.go and check a build fails
      even without TARGET_VNDK_USE_CORE_VARIANT set.

Change-Id: I7698edf9a24b0df150c0e4f7d8dd4926f053eee8

cc/androidmk.go

@@ -217,6 +217,9 @@ func (library *libraryDecorator) AndroidMk(ctx AndroidMkContext, ret *android.An
 			fmt.Fprintln(w, "LOCAL_NO_NOTICE_FILE := true")
 			fmt.Fprintln(w, "LOCAL_VNDK_DEPEND_ON_CORE_VARIANT := true")
 		}
+		if library.checkSameCoreVariant {
+			fmt.Fprintln(w, "LOCAL_CHECK_SAME_VNDK_VARIANTS := true")
+		}
 	})
 
 	if library.shared() && !library.buildStubs() {