Fix HOST_PREFER_32_BITS tests

When HOST_PREFER_32_BITS is set, HOST_ARCH is still x86_64 but art tests
are only built for x86.  Use ART_HOST_ARCH, which will be set to x86,
to find the list of host tests.

Test: m -j HOST_PREFER_32_BITS=true test-art-host-gtest32
Change-Id: I11a9e1b5111c3c06139b44e364fdcd04bb8db4c6
diff --git a/build/Android.gtest.mk b/build/Android.gtest.mk
index f73f499..b93c1af 100644
--- a/build/Android.gtest.mk
+++ b/build/Android.gtest.mk
@@ -199,7 +199,7 @@
     $(ART_TEST_LIST_device_$(TARGET_ARCH)_$(m)))
 
 ART_HOST_GTEST_FILES := $(foreach m,$(ART_TEST_MODULES),\
-    $(ART_TEST_LIST_host_$(HOST_ARCH)_$(m)))
+    $(ART_TEST_LIST_host_$(ART_HOST_ARCH)_$(m)))
 
 ART_TEST_CFLAGS :=