Rollup merge of #74419 - Lokathor:gba-target, r=jonas-schievink

Add a thumbv4t-none-eabi target

(cc @ketsuban, one of the few other Rust users who programs for GBA.)

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**EDIT:** This is now a more general `thumbv4t-none-eabi` PR! See [this comment](https://github.com/rust-lang/rust/pull/74419#issuecomment-660391579)

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Now that the PSP officially has an official target within Rust, well as the lead of the `gba` crate I can't _not_ add a GBA target as well.

I know that the [target tier policy](https://github.com/rust-lang/rfcs/pull/2803) isn't ratified and official, but I'll use it as an outline (cc @joshtriplett):
* Designated Developer: Lokathor
* Naming consistent with any existing targets
* Doesn't create Rust project legal issues.
* No license issues
* Uses the standard Apache/mit license.
* Rust tooling users don't have to accept any new licensing requirements
* Does not support hosting rust tooling.
* Doesn't require linking in proprietary code to obtain a functional binary. However, you will need to do some post-build steps to turn the ELF file into a usable GBA ROM (either for an emulator or for the actual hardware).
* This is a `no_std` environment, without even a standard global allocator, so this adds no new code to `alloc` or `std`.
* The process of building for this target is documented in the `gba` crate ([link](https://rust-console.github.io/gba/development-setup.html)). Well, the docs there are currently a little out of date, they're back on using `cargo-xbuild`, but the crate docs there will get updated once this target is available.
* This places no new burden on any other targets
* Does not break any existing targets.

I'm not fully confident in specifying the same linker script for all possible projects, so I'm currently just not giving a linker script at all, and users can continue to select their own linker script by using `-C` to provide a linker arg.

I added the file, and added it to the `supported_targets!` macro usage, and I think that's all there is to do.

diff --cc src/librustc_target/spec/mod.rs
Simple merge