Support TARGET_ARCH=64 with no 2ND_TARGET_ARCH.

This saves build time for the target defined in device/generic/armv8,
by only building 64bits bin and lib.

Change-Id: Ie2ad1411c127e60ca7569b4cbf4c0b52a4382d3e
diff --git a/build/Android.common.mk b/build/Android.common.mk
index 39a734d..39e78fa 100644
--- a/build/Android.common.mk
+++ b/build/Android.common.mk
@@ -42,10 +42,17 @@
     $(error Do not know what to do with this multi-target configuration!)
   endif
 else
-  ART_PHONY_TEST_TARGET_SUFFIX := 32
-  2ND_ART_PHONY_TEST_TARGET_SUFFIX :=
-  ART_TARGET_ARCH_32 := $(TARGET_ARCH)
-  ART_TARGET_ARCH_64 :=
+  ifneq ($(filter %64,$(TARGET_ARCH)),)
+    ART_PHONY_TEST_TARGET_SUFFIX := 64
+    2ND_ART_PHONY_TEST_TARGET_SUFFIX :=
+    ART_TARGET_ARCH_32 :=
+    ART_TARGET_ARCH_64 := $(TARGET_ARCH)
+  else
+    ART_PHONY_TEST_TARGET_SUFFIX := 32
+    2ND_ART_PHONY_TEST_TARGET_SUFFIX :=
+    ART_TARGET_ARCH_32 := $(TARGET_ARCH)
+    ART_TARGET_ARCH_64 :=
+  endif
 endif
 
 ART_HOST_SHLIB_EXTENSION := $(HOST_SHLIB_SUFFIX)