kbuild: buildtar: add arm64 support

Make 'make tar-pkg' work on arm64.

Cc: Will Deacon <will.deacon@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>