bootstrap: illumos platform flags for split-debuginfo

[citrus-it]

Bootstrap currently provides -Zunstable-options for platforms
when using split debuginfo - this commit adds it for the illumos
target too.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Mark-Simulacrum (or someone else) soon.

Please see the contribution instructions for more information.

[citrus-it]

For reference, here is what I see when doing a build of rust 1.62.0 on illumos, without this patch:

error: failed to run `rustc` to learn about target-specific information

Caused by:
  process didn't exit successfully: `/data/omnios-build/omniosorg/r151042/_extra/rust-1.62.0/rustc-1.62.0-src/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-illumos --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)
  --- stderr
  error: `-Csplit-debuginfo` is unstable on this platform

[Mark-Simulacrum]

@bors r+ rollup

[bors]

📌 Commit 5122bb5 has been approved by Mark-Simulacrum