Documentation/llvm: Add a note about prebuilt kernel.org toolchains

Documentation/llvm: Add a note about prebuilt  toolchains

I recently started uploading prebuilt stable versions of LLVM to
kernel.org, which should make building the kernel with LLVM more
accessible to maintainers and developers. Link them in the LLVM
documentation to make this more visible.

Link: https://lore.kernel.org/20230319235619.GA18547@dev-arch.thelio-3990X/
Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Bill Wendling <morbo@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>