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

[NAKAMURA Takumi via cfe-commits]

chapuni 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`.

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