kbuild: initialize CLANG_FLAGS correctly in the top Makefile

CLANG_FLAGS is initialized by the following line:

  CLANG_FLAGS     := --target=$(notdir $(CROSS_COMPILE:%-=%))

..., which is run only when CROSS_COMPILE is set.

Some build targets (bindeb-pkg etc.) recurse to the top Makefile.

When you build the kernel with Clang but without CROSS_COMPILE,
the same compiler flags such as -no-integrated-as are accumulated
into CLANG_FLAGS.

If you run 'make CC=clang' and then 'make CC=clang bindeb-pkg',
Kbuild will recompile everything needlessly due to the build command
change.

Fix this by correctly initializing CLANG_FLAGS.

Fixes: 238bcbc4

 ("kbuild: consolidate Clang compiler flags")
Cc: <stable@vger.kernel.org> # v5.0+
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Acked-by: Nick Desaulniers <ndesaulniers@google.com>