Skip make targets that have missing dependencies when using ART
prebuilts.

This is necessary because ART sources can still be in the tree when
prebuilts are used, and e.g. for host we don't have a boot image in
them.

Test: m checkbuild
  with and without SOONG_CONFIG_art_module_source_build=false, check
  that it passes the "Test for dangling rules" stage
Bug: 172480615
Change-Id: I4f24a430d1b11202eb4dceeedc96a0906679830e