Create armv4t_none_eabi.md

src/doc/rustc/src/platform-support/armv4t_none_eabi.md

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+# armv4t-none-eabi
+
+Tier 3
+
+Bare-metal target for any cpu in the ARMv4T architecture family, supporting
+ARM/Thumb code interworking (aka `a32`/`t32`), with ARM code as the default code
+generation.
+
+In particular this supports the Gameboy Advance (GBA), but there's nothing GBA
+specific with this target, so any ARMv4T device should work fine.
+
+## Target Maintainers
+
+* [@Lokathor](https://github.com/lokathor)
+
+## Requirements
+
+The target is cross-compiled, and uses static linking.
+
+The linker that comes with rustc cannot link for this platform (the platform is
+too old). You will need the `arm-none-eabi-ld` linker from a GNU Binutils
+targeting ARM. This can be obtained for Windows/Mac/Linux from the [ARM
+Developer Website][arm-dev], or possibly from your OS's package manager.
+
+[arm-dev]: https://developer.arm.com/Tools%20and%20Software/GNU%20Toolchain
+
+This target doesn't provide a linker script, you'll need to bring your own
+according to the specific device you want to target. Pass
+`-Clink-arg=-Tyour_script.ld` as a rustc argument to make the linker use
+`your_script.ld` during linking.
+
+## Building Rust Programs
+
+Because it is Tier 3, rust does not yet ship pre-compiled artifacts for this target.
+
+Just use the `build-std` nightly cargo feature to build the `core` library. You
+can pass this as a command line argument to cargo, or your `.cargo/config.toml`
+file might include the following lines:
+
+```toml
+[unstable]
+build-std = ["core"]
+```
+
+Most of `core` should work as expected, with the following notes:
+* the target is "soft float", so `f32` and `f64` operations are emulated in
+  software.
+* integer division is also emulated in software.
+* the target is old enough that it doesn't have atomic instructions.
+
+Rust programs are output as ELF files.
+
+For running on hardware, you'll generally need to extract the "raw" program code
+out of the ELF and into a file of its own. The `objcopy` program provided as
+part of the GNU Binutils can do this:
+
+```shell
+arm-none-eabi-objcopy --output-target binary [in_file] [out_file]
+```
+
+## Testing
+
+This is a cross-compiled target that you will need to emulate during testing.
+
+Because this is a device-agnostic target, and the exact emulator that you'll
+need depends on the specific device you want to run your code on.
+
+For example, when programming for the Gameboy Advance, the
+[mgba-test-runner](https://github.com/agbrs/agb) program could be used to make a
+normal set of rust tests be run within the `mgba` emulator.