kbuild: make clean rule robust against too long argument error

Commit cd968b97 ("kbuild: make built-in.a rule robust against too
long argument error") made a build rule robust against "Argument list
too long" error.

Eugeniu Rosca reported the same error occurred when cleaning an external
module.

The $(obj)/ prefix can be a very long path for external modules.

Apply a similar solution to 'make clean'.

Reported-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Tested-by: Eugeniu Rosca <erosca@de.adit-jv.com>