Restore dummy libdl.a implementation

Bug: http://b/36206043
Test: bionic-unit-tests-static
Change-Id: If0400b72b51570e5ca0d70ec0bd9fb77664b9210
diff --git a/libdl/Android.bp b/libdl/Android.bp
index ffdf7f7..9f67ee8 100644
--- a/libdl/Android.bp
+++ b/libdl/Android.bp
@@ -44,7 +44,12 @@
             version_script: "libdl.x86_64.map",
         },
     },
-    srcs: ["libdl.c", "libdl_cfi.cpp"],
+    shared: {
+        srcs: ["libdl.c", "libdl_cfi.cpp"],
+    },
+    static: {
+        srcs: [ "libdl_static.c" ],
+    },
     cflags: [
         "-Wall",
         "-Wextra",