commit fcf1b6a35c16ac500fa908a4022238e5d666eabf upstream.

added to kbuild documentation. Provides more official info on building
kernels with Clang and LLVM than our wiki.

Suggested-by: Kees Cook
Reviewed-by: Kees Cook
Reviewed-by: Nathan Chancellor
Reviewed-by: Sedat Dilek
Signed-off-by: Nick Desaulniers
Signed-off-by: Masahiro Yamada
Signed-off-by: Greg Kroah-Hartman