Add a gtest build target for the host.

Change-Id: Ib888e143c7f3c110a0596ce37f51138a4da31b5a
http://b/4584343
diff --git a/Android.common.mk b/Android.common.mk
index 69ea22c..496dfa3 100644
--- a/Android.common.mk
+++ b/Android.common.mk
@@ -33,3 +33,7 @@
 LIBART_LOCAL_SRC_FILES += \
 	src/assembler_x86.cc
 endif
+
+TEST_LOCAL_SRC_FILES := \
+	src/assembler_x86_test.cc
+