[lld] [LLD] Extend special OpenBSD support, but scope under ELFOSABI (PR #97122)

[John Ericson via llvm-commits]

Ericson2314 wrote:

Linux CI is stuck in the queue, Windows has
```
PASS: lld :: ELF/openbsd-phdr.s (1928 of 2949)
...
PASS: lld :: ELF/relro-openbsd.s (2129 of 2949)
PASS: lld :: ELF/relro.s (2130 of 2949)
```
and there is no thing host-specific about this, so I think we are good!

https://github.com/llvm/llvm-project/pull/97122