kbuild: clang: fix build failures with sparse check

We should avoid using the space character when passing arguments to
clang, because static code analysis check tool such as sparse may
misinterpret the arguments followed by spaces as build targets hence
cause the build to fail.

Signed-off-by: David Lin <dtwlin@google.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>