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

[Fangrui Song via llvm-commits]

================
@@ -0,0 +1,2 @@
+# RUN: llvm-mc -filetype obj -triple amd64-freebsd %s | llvm-readobj -hS - | FileCheck %s
----------------
MaskRay wrote:

I added `osabi.s` to unify FreeBSD/Linux/Solaris testing. Consider adding OpenBSD tests there instead of adding a new file.

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