Document minimum supported versions of Solaris and Illumos

Location

https://doc.rust-lang.org/stable/rustc/platform-support.html

Summary

Rust currently supports multiple Illumos and Solaris targets (see Platform Support), but other than saying Rust supports Solaris 11, it doesn't clarify which versions of Illumos and Solaris are supported.

For context, https://github.com/rust-random/getrandom aims to supports all targets/platforms that Rust supports, including Illumos/Solaris. We currently have fallback logic to support old (pre 2018) Solaris/Illumos versions, and we are wondering when it would be OK to remove those fallbacks (both from getrandom and the standard library).

I would propose a minimum platform version of:

• Solaris 11.3 (released October 2015)
• Illumos 0.29 (released 2018)

This would allow the standard library to unconditionally use the getrandom(2) syscall and unconditionally use named threads.

I'm not sure who's in charge of this decision, maybe @psumbera or @Toasterson

Open rustc/platform-support.html and inspect the existing Solaris and Illumos target entries, then review the linked Solaris, Illumos, getrandom, and standard-library references. Confirm the accepted minimum versions with the relevant maintainers before editing. Done means the platform-support documentation clearly states the minimum supported Solaris and Illumos versions.