bionic: rename aarch64 target to arm64

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

Change-Id: Ia92d8a50824e5329cf00fd6f4f92eae112b7f3a3
diff --git a/libm/Android.mk b/libm/Android.mk
index 1c3d393..ce98236 100644
--- a/libm/Android.mk
+++ b/libm/Android.mk
@@ -225,12 +225,12 @@
 
 libm_common_includes := $(LOCAL_PATH)/upstream-freebsd/lib/msun/src/
 
-libm_aarch64_includes := $(LOCAL_PATH)/aarch64
-libm_aarch64_src_files := aarch64/fenv.c
-
 libm_arm_includes := $(LOCAL_PATH)/arm
 libm_arm_src_files := arm/fenv.c
 
+libm_arm64_includes := $(LOCAL_PATH)/arm64
+libm_arm64_src_files := arm64/fenv.c
+
 libm_x86_includes := $(LOCAL_PATH)/i386 $(LOCAL_PATH)/i387
 libm_x86_src_files := i387/fenv.c