BACKPORT: Documentation/llvm: add documentation on building w/ Clang/LLVM

BACKPORT: Documentation/llvm: add documentation on building w/ Clang/LLVM

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

Suggested-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
(cherry picked from commit fcf1b6a35c16ac500fa908a4022238e5d666eabf)
Change-Id: I33bc4a97cf9e9ef923712bb490df0bceac81dade