Rollup merge of #96597 - semarie:split_debuginfo-unix, r=davidtwco,Mark-Simulacrum

openbsd: unbreak build on native platform

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

currently, without this PR, the build of rustc on OpenBSD fails with:

```
Building stage0 tool unstable-book-gen (x86_64-unknown-openbsd)
running: "/data/semarie/build-rust/install_dir/beta/bin/cargo" "build" "--target" "x86_64-unknown-openbsd" "-Zbinary-dep-depinfo" "-j" "4" "-v" "--release" "--frozen" "--manifest-path"
"/data/semarie/build-rust/build_dir/rustc-nightly-src/src/tools/unstable-book-gen/Cargo.toml" "--message-format" "json-render-diagnostics"
error: failed to run `rustc` to learn about target-specific information

Caused by:
  process didn't exit successfully: `/data/semarie/build-rust/build_dir/build/bootstrap/debug/rustc - --crate-name ___ --print=file-names --cfg=bootstrap -Csymbol-mangling-version=v0 -Zmacro-backtrace -Clink-args=-Wl,-z,origin
'-Clink-args=-Wl,-rpath,$ORIGIN/../lib' -Csplit-debuginfo=off -Ztls-model=initial-exec --target x86_64-unknown-openbsd --crate-type bin --crate-type rlib --crate-type dylib --crate-type cdylib --crate-type staticlib --crate-type proc-macro
--print=sysroot --print=cfg` (exit status: 1)
  --- stdout

  Did not run successfully: exit status: 1
  "/data/semarie/build-rust/install_dir/beta/bin/rustc" "-" "--crate-name" "___" "--print=file-names" "--cfg=bootstrap" "-Csymbol-mangling-version=v0" "-Zmacro-backtrace" "-Clink-args=-Wl,-z,origin" "-Clink-args=-Wl,-rpath,$ORIGIN/../lib"
"-Csplit-debuginfo=off" "-Ztls-model=initial-exec" "--target" "x86_64-unknown-openbsd" "--crate-type" "bin" "--crate-type" "rlib" "--crate-type" "dylib" "--crate-type" "cdylib" "--crate-type" "staticlib" "--crate-type" "proc-macro"
"--print=sysroot" "--print=cfg" "-Wrust_2018_idioms" "-Wunused_lifetimes" "-Wsemicolon_in_expressions_from_macros" "-Dwarnings" "--sysroot" "/data/semarie/build-rust/install_dir/beta"
  -------------

  --- stderr
  error: `-Csplit-debuginfo` is unstable on this platform

command did not execute successfully: "/data/semarie/build-rust/install_dir/beta/bin/cargo.bin" "build" "--target" "x86_64-unknown-openbsd" "-Zbinary-dep-depinfo" "-j" "4" "-v" "--release" "--frozen" "--manifest-path"
```

I am suspecting that all unix might be affected, but I am unsure about the right conditional to use. so I only added "openbsd" target inside it as I am able to test it.

rustc nightly built correctly with this PR on openbsd.

diff --cc src/bootstrap/builder.rs
Simple merge