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
diff --git a/libnativeloader/native_loader_namespace.cpp b/libnativeloader/native_loader_namespace.cpp
index 49f3035..79a9791 100644
--- a/libnativeloader/native_loader_namespace.cpp
+++ b/libnativeloader/native_loader_namespace.cpp
@@ -14,6 +14,8 @@
  * limitations under the License.
  */
 
+#if defined(ART_TARGET_ANDROID)
+
 #define LOG_TAG "nativeloader"
 
 #include "native_loader_namespace.h"
@@ -175,3 +177,5 @@
 }
 
 }  // namespace android
+
+#endif  // defined(ART_TARGET_ANDROID)