[lld] [lld][MachO] Support for -interposable (PR #131813)

[Shoaib Meenai via llvm-commits]

smeenai wrote:

`llvm-objdump` will always be available, so we can use that. For the test, I found that using `--macho` alongside `-d` gets it to print the stub information directly, which we can take advantage of:

```
BUNDLE-OBJ-LABEL: my_user:
BUNDLE-OBJ-NEXT:          callq   [[#%#x,]] ## symbol stub for: my_func
```

That's using a [numeric substitution block](https://llvm.org/docs/CommandGuide/FileCheck.html#filecheck-numeric-substitution-blocks) to capture the hex value, and it's also taking advantage of FileCheck doing partial matches on lines by default (so you don't need to specify the entire contents of a line to match against, just the bits you care about).

I'd also suggest renaming the functions to `_my_user` and `_my_friend`, since the leading underscore is user label prefix on Mach-O.

https://github.com/llvm/llvm-project/pull/131813