Makefile: Export clang toolchain variables

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>

Makefile

@@ -498,6 +498,7 @@ endif
 CLANG_FLAGS	+= -no-integrated-as
 KBUILD_CFLAGS	+= $(CLANG_FLAGS)
 KBUILD_AFLAGS	+= $(CLANG_FLAGS)
+export CLANG_FLAGS
 endif
 
 RETPOLINE_CFLAGS_GCC := -mindirect-branch=thunk-extern -mindirect-branch-register