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/library_namespaces.h b/libnativeloader/library_namespaces.h
index e6d1a87..c41f2e4 100644
--- a/libnativeloader/library_namespaces.h
+++ b/libnativeloader/library_namespaces.h
@@ -17,8 +17,8 @@
 #ifndef ART_LIBNATIVELOADER_LIBRARY_NAMESPACES_H_
 #define ART_LIBNATIVELOADER_LIBRARY_NAMESPACES_H_
 
-#if !defined(__ANDROID__)
-#error "Not available for host"
+#if !defined(ART_TARGET_ANDROID)
+#error "Not available for host or linux target"
 #endif
 
 #define LOG_TAG "nativeloader"