openbsd: unbreak build on native platform

after rust-lang#95612, only linux and windows target are build with -Zunstable-options, 
but others platforms might use -Csplit-debuginfo

add "openbsd" target in the list of platforms using it.

Author: semarie

src/bootstrap/builder.rs

@@ -1406,7 +1406,8 @@ impl<'a> Builder<'a> {
         // FIXME(davidtwco): #[cfg(not(bootstrap))] - #95612 needs to be in the bootstrap compiler
         // for this conditional to be removed.
         if !target.contains("windows") || compiler.stage >= 1 {
-            if target.contains("linux") || target.contains("windows") {
+            if target.contains("linux") || target.contains("windows") || target.contains("openbsd")
+            {
                 rustflags.arg("-Zunstable-options");
             }
             match self.config.rust_split_debuginfo {