doc/platform-support: Add documentation for m68k-unknown-linux-gnu

src/doc/rustc/src/platform-support/m68k-unknown-linux-gnu.md

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+# m68k-unknown-linux-gnu
+
+**Tier: 3**
+
+Motorola 680x0 Linux
+
+## Designated Developers
+
+* [@glaubitz](https://github.com/glaubitz)
+* [@ricky26](https://github.com/ricky26)
+
+## Requirements
+
+This target requires a Linux/m68k build environment for cross-compilation which
+is available on Debian and Debian-based systems, openSUSE and other distributions.
+
+On Debian, it should be sufficient to install a g++ cross-compiler for the m68k
+architecture which will automatically pull in additional dependencies such as
+the glibc cross development package:
+
+```text
+# apt install g++-m68k-linux-gnu
+```
+
+Binaries can be run using QEMU user emulation. On Debian-based systems, it should be
+sufficient to install the package `qemu-user-static` to be able to run simple static
+binaries:
+
+```text
+# apt install qemu-user-static
+```
+
+To run more complex programs, it will be necessary to set up a Debian/m68k chroot with
+the help of the command `debootstrap`:
+
+```text
+# apt install debootstrap debian-ports-archive-keyring
+# debootstrap --keyring=/usr/share/keyrings/debian-ports-archive-keyring.gpg --arch=m68k unstable debian-68k http://ftp.ports.debian.org/debian-ports
+```
+
+This chroot can then seamlessly entered using the normal `chroot` command thanks to
+QEMU user emulation:
+
+```text
+# chroot /path/to/debian-68k
+```
+
+To get started with native builds, which are currently untested, a native Debian/m68k
+system can be installed either on real hardware such as 68k-based Commodore Amiga or
+Atari systems or emulated environments such as QEMU version 4.2 or newer or ARAnyM.
+
+ISO images for installation are provided by the Debian Ports team and can be obtained
+from the Debian CD image server available at:
+
+[https://cdimage.debian.org/cdimage/ports/current](https://cdimage.debian.org/cdimage/ports/current/)
+
+Documentation for Debian/m68k is available on the Debian Wiki at:
+
+[https://wiki.debian.org/M68k](https://wiki.debian.org/M68k)
+
+Support is available either through the `debian-68k` mailing list:
+
+[https://lists.debian.org/debian-68k/](https://lists.debian.org/debian-68k/)
+
+or the `#debian-68k` IRC channel on OFTC network.
+
+## Building
+
+The codegen for this target should be built by default. However, core and std
+are currently missing but are being worked on and should become available in
+the near future.
+
+## Cross-compilation
+
+This target can be cross-compiled from a standard Debian or Debian-based, openSUSE or any
+other distribution which has a basic m68k cross-toolchain available.
+
+## Testing
+
+Currently there is no support to run the rustc test suite for this target.
+
+## Building Rust programs
+
+Rust programs can be built for that target:
+
+```text
+rustc --target m68k-unknown-linux-gnu your-code.rs
+```
+
+Very simple progams can be run using the `qemu-m68k-static` program:
+
+```text
+$ qemu-m68k-static your-code
+```
+
+For more complex applications, a chroot or native (emulated) Debian/m68k system are required
+for testing.