bionic: rename aarch64 target to arm64

Rename aarch64 build targets to arm64.  The gcc toolchain is still
aarch64.

Change-Id: Ia92d8a50824e5329cf00fd6f4f92eae112b7f3a3
diff --git a/tests/Android.mk b/tests/Android.mk
index 0540400..4985643 100644
--- a/tests/Android.mk
+++ b/tests/Android.mk
@@ -29,10 +29,10 @@
     -Werror \
     -fno-builtin \
 
-ifeq ($(TARGET_ARCH),aarch64)
+ifeq ($(TARGET_ARCH),arm64)
   $(info TODO: $(LOCAL_PATH)/Android.mk -fstack-protector not yet available for the AArch64 toolchain)
   test_c_flags += -fno-stack-protector
-endif # aarch64
+endif # arm64
 
 test_src_files = \
     buffer_tests.cpp \