[clang] [SystemZ][z/OS] Add guard for dl_info and dladdr (PR #75637)

[Abhina Sree via cfe-commits]

abhina-sree wrote:

> Could we take other ways? I think they don't affect external API.
> 
> 1. Detect stuff in `clang/Config/config.h`
> 2. Migrate the special logic to `llvm/Support`.

Thanks, I've created a patch here that detects the header in clang's config.h. Please let me know if this is a good approach

https://github.com/llvm/llvm-project/pull/75637