Add illumos target documentation

Author: Noratrieb

src/doc/rustc/src/platform-support/illumos.md

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+# `aarch64-unknown-illumos` and `x86_64-unknown-illumos`
+
+**Tier: 2/3**
+
+[illumos](https://www.illumos.org/), is a Unix operating system which provides next-generation features for downstream distributions,
+including advanced system debugging, next generation filesystem, networking, and virtualization options.
+
+## Target maintainers
+
+- Joshua M. Clulow ([@jclulow](https://github.com/jclulow))
+- Patrick Mooney ([@pfmooney](https://github.com/pfmooney))
+
+## Requirements
+
+The target supports host tools.
+
+The illumos target supports `std` and uses the standard ELF file format.
+
+`x86_64-unknown-illumos` is a tier 2 target with host tools.
+`aarch64-unknown-illumos` is a tier 3 target.
+
+## Building the target
+
+These targets can be built by adding `aarch64-unknown-illumos` and
+`x86_64-unknown-illumos` as targets in the rustc list.
+
+## Building Rust programs
+
+Rust ships pre-compiled artifacts for the `x86_64-unknown-illumos` target.
+Rust does not ship pre-compiled artifacts for `aarch64-unknown-illumos`,
+it requires building the target either as shown above or using `-Zbuild-std`.
+
+## Testing
+
+Tests can be run in the same way as a regular binary.
+
+## Cross-compilation toolchains and C code
+
+The target supports C code.
+
+The illumos project makes available [prebuilt sysroot artefacts](https://github.com/illumos/sysroot) which can be used for cross compilation.
+The official Rust binaries are cross-compiled using these artefacts.