ac/llvm: handle static/shared llvm init separately

ac/llvm: handle static/shared llvm init separately

Having a single init function works as expected for shared llvm, but
when using a static llvm only one llvm will get initialized.

This commit introduces 2 separate init function:
- shared llvm = single public init function
- static llvm = one init function for each module using llvm

Fixes: 50d20dc055d ("ac/llvm: export ac_init_llvm_once in targets")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/3376
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6253>