Create empty .a on Darwin when there is no obj file.

On Darwin ar would fail if there is no object file to add.
We work around by adding a dummy.o.

Port to Soong of build/ 4aaa1a1fd8e7eb23ac5557cd326d1a48efdb54cd

Change-Id: I68bbebea2726058c25863d7026a645a520d05167
1 file changed