Add a test for Mach-O #[link_section] API inherited from LLVM

[madsmtm]

The format of the #[link_section] attribute is under-documented, but on Mach-O, I think it's roughly the following BNF:

LinkSection -> Segment `,` Section (`,` (SectionType (`,` (SectionAttributes)?)?)?)?
Segment -> <0 to 16 bytes>
Section -> <0 to 16 bytes>
SectionType -> `regular` | `zerofill` | `cstring_literals` | `4byte_literals` | `8byte_literals` | `literal_pointers` | `non_lazy_symbol_pointers` | `lazy_symbol_pointers` | `symbol_stubs` | `mod_init_funcs` | `mod_term_funcs` | `coalesced` | `interposing` | `16byte_literals` | `thread_local_regular` | `thread_local_zerofill` | `thread_local_variables` | `thread_local_variable_pointers` | `thread_local_init_function_pointers`
SectionAttributes -> SectionAttribute (`+` SectionAttribute)*
SectionAttribute -> `pure_instructions` | `no_toc` | `strip_static_syms` | `no_dead_strip` | `live_support`, `self_modifying_code` | `debug`

This PR adds a small test for a little part of this.

Once #154429 is resolved, this should make it possible to test rust-lang/rustc_codegen_cranelift#1648 end-to-end.

r? bjorn3