Add more information to the ABI error message.

Just knowing it failed is pretty useless. Tell people how to update
the ABI dumps.

Bug: http://b/156513478#comment43
Test: m ndk # after breaking the ABI
Change-Id: I7ed6a0acf2d1fbd942410678fc6aba2afbc5a2b6

cc/ndk_library.go

@@ -61,7 +61,7 @@ var (
 			// because we don't want to spam the build output with "nothing
 			// changed" messages, so redirect output message to $out, and if
 			// changes were detected print the output and fail.
-			Command:     "$stgdiff $args --stg $in -o $out || (cat $out && false)",
+			Command:     "$stgdiff $args --stg $in -o $out || (cat $out && false; echo 'Run $$ANDROID_BUILD_TOP/development/tools/ndk/update_ndk_abi.sh to update the ABI dumps.')",
 			CommandDeps: []string{"$stgdiff"},
 		}, "args")