llvm 21 incoming soon

Hi!

Just a heads-up:

I've updated llvm & friends in pkgsrc to version 21 locally.  Now that
zig 0.16 is out, all the packages in bulk-test-llvm built fine for me
on 11.99.5/x86_64, so I will be committing this update shortly.

This is mostly based on adam@'s wip packages (which are already at
version 22 though, but that version is not well-supported enough yet
to update to).

Cheers,
 Thomas