Mark bionic-loader-test-libs-target as a phony rule

This essentially was phony before, but it wasn't marked as such. Mark it
so that we can turn on some more sanity checks in the build.

Test: Fewer warnings when --warn_real_to_phony is enabled for kati
Change-Id: I5f02b291ca01141dea75977df8b5ef9630917eb2