[clang] [HIP][MacOS] Mach-O support and Darwin toolchain fixes (PR #183991)

[Joseph Huber via cfe-commits]

https://github.com/jhuber6 commented:

If you want Darwin support you'll also need to modify how the offloading entries are managed. But I believe that's only for RDC compilation. For that you'd need to follow the same runtime array handling that compiler-rt uses.

Something like this,
```
  ELF / COFF:  llvm_offload_entries
  Mach-O:      __LLVM,offload_entries
```

And then in the Offloading utilities to create the entires you'd need to make a dummy variable like ELF and reference the start and end.
```
\1section$start$__LLVM$offload_entries
\1section$end$__LLVM$offload_entries
```

https://github.com/llvm/llvm-project/pull/183991