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

Documentation/kbuild/llvm.rst

@@ -171,6 +171,10 @@ Getting Help
 Getting LLVM
 -------------
 
+We provide prebuilt stable versions of LLVM on `kernel.org <https://kernel.org/pub/tools/llvm/>`_.
+Below are links that may be useful for building LLVM from source or procuring
+it through a distribution's package manager.
+
 - https://releases.llvm.org/download.html
 - https://github.com/llvm/llvm-project
 - https://llvm.org/docs/GettingStarted.html