[llvm] [LLVM] set OpenBSD's ELFOSABI by default (PR #98158)

[John Ericson via llvm-commits]

Ericson2314 wrote:

Note to self: https://github.com/llvm/llvm-project/pull/97122#discussion_r1671426444 applies to this PR, since I split it.

https://github.com/llvm/llvm-project/pull/98158