Use ART_TARGET_ANDROID instead of __ANDROID__

On golem, we build for ART_TARGET_LINUX, which doesn't have support for
linkerconfig. Therefore, guard all linker namespace code on
ART_TARGET_ANDROID instead of __ANDROID__ (which is unfortunately not
under ART's control).

Test: build for golem and run
Bug: 154074847
Change-Id: I7a2b81918177704b42d8aafbd6d7e9d06d34e5f4
7 files changed