Makefile: Export clang toolchain variables

commit 3bd9805090af843b25f97ffe5049f20ade1d86d6 upstream.

The powerpc makefile will use these in it's boot wrapper.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>