[compiler-rt] [compiler-rt] Fix building on OpenBSD/amd64 (PR #165086)

[Oliver Hunt via llvm-commits]

https://github.com/ojhunt requested changes to this pull request.

I'm not sure about this.

If the pointer size conditions really are necessary, then adding the `OPENBSD` definition seems like a better approach, although perhaps a better solution would be to work out why `test_target_arch` is not working on OBSD, and fixing that test macro.

If the problem with the test macro are resolvable, then it might be ok to retain the `(NOT STREQUAL)` version, but I would be perfectly ok with adding an OPENBSD definition and using that, though open to dissent from others :D

https://github.com/llvm/llvm-project/pull/165086