system/core: rename aarch64 target to arm64

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

Change-Id: Ia92d8a50824e5329cf00fd6f4f92eae112b7f3a3
diff --git a/libbacktrace/Android.mk b/libbacktrace/Android.mk
index d729c1c..9c52ad7 100644
--- a/libbacktrace/Android.mk
+++ b/libbacktrace/Android.mk
@@ -148,10 +148,10 @@
 	-DGTEST_OS_LINUX_ANDROID \
 	-DGTEST_HAS_STD_STRING \
 
-ifeq ($(TARGET_ARCH),aarch64)
+ifeq ($(TARGET_ARCH),arm64)
   $(info TODO: $(LOCAL_PATH)/Android.mk -fstack-protector not yet available for the AArch64 toolchain)
   LOCAL_CFLAGS += -fno-stack-protector
-endif # aarch64
+endif # arm64
 
 LOCAL_CONLYFLAGS += \
 	$(common_conlyflags) \