`#[used(linker)]` is broken on illumos

On illumos, #[used(linker)] generates the section flag 0x100000, which is known as SHF_SUNW_NODISCARD on Solaris, but unknown to illumos. This causes incorrect behavior when used in combination with .init_array, for example by inventory.

This regressed in https://github.com/rust-lang/rust/pull/140872 which made #[used] mean #[used(linker)] on all ELF targets, including illumos.

See

• Original investigation by @MarceColl on this issue: https://system-illumination.org/01-rustler.html
  • discussed on lobste.rs: https://lobste.rs/s/nr8kkd/debugging_rustler_on_illumos
  • Attempt to work-around in the inventory crate: https://github.com/dtolnay/inventory/pull/84
• LLVM change to start using SHF_SUNW_NODISCARD on Solaris: https://reviews.llvm.org/D107955#:~:text=One,them
• LLVM change related to retained section behavior: https://reviews.llvm.org/D97448

Start by reviewing rust-lang/rust#140872 and the handling of #[used(linker)] for ELF targets, then compare illumos with Solaris and the LLVM changes linked in the issue. Check the inventory workaround and retained-section behavior; done means #[used(linker)] no longer produces incorrect .init_array behavior on illumos.