ARM: disable HAVE_PRIVATE_LIBGCC for ARM64

We have not supported the private library for ARM 64bit.
Prohibit ARM64 boards from enabling it until we make things ready.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

arch/Kconfig

@@ -18,7 +18,7 @@ config ARC
 
 config ARM
 	bool "ARM architecture"
-	select HAVE_PRIVATE_LIBGCC
+	select HAVE_PRIVATE_LIBGCC if !ARM64
 	select HAVE_GENERIC_BOARD
 	select SUPPORT_OF_CONTROL