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

[John Ericson via llvm-commits]

Ericson2314 wrote:

OK @MaskRay I think this is ready now?

- (Previously) addressed your 3 prior review comments
- Rebased atop merged #98158 so it's just the one commit
- changed `-triple` to be `x86_64-unknown-freebsd` (with `unknown`) to match what you did in 06cecdc60ec9ebfdd4d8cdb2586d201272bdf6bd

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