rust packages: clang dependency: bl3.mk -> TOOL

Hi!

We have quite a few packages that include lang/clang/buildlink3.mk.

Today I wanted to know why that is needed and looked at one of them,
wip/codex.  It uses bindgen = bindgen-rs = rust-bindgen, which uses
the clang library when parsing C/C++ header files.

However, bindgen is a build time tool. It generates binding files
which are then used to build something interfacing with a C/C++
library. This is not a run-time dependency.

So please, when updating rust packages and you see
lang/clang/buildlink3.mk, try replacing it with

TOOL_DEPENDS+= clang-[0-9]*:../../lang/clang

instead.

Thanks,
 Thomas