ci: add a Linux aarch64 build

[WizzardSK]

Adds a libretro-build-linux-aarch64 job so the core lands in nightly/linux/aarch64/ on the buildbot, where it is currently missing.

Why it is not a one-liner

There is no rust-linux-aarch64.yml template in ci-templates — only rust-linux-x64.yml — and libretro-build-rust is FROM libretro-build-amd64-ubuntu, so there is no native aarch64 image to point at either. I raised that gap as libretro/libretro-super#2030.

So this job cross-compiles from the same image the x64 job already uses, in the style that image already uses for its other arm64 targets (CARGO_TARGET_AARCH64_APPLE_DARWIN_LINKER and friends): add the target, point CARGO_TARGET_AARCH64_UNKNOWN_LINUX_GNU_LINKER at aarch64-linux-gnu-gcc, and override STRIP, since the template's default strip is host binutils (rust-webos.yml does the same for its ARM targets). .libretro-rust-linux-x64 is extended rather than -default, since that is what pins TARGET_ARCH to x86_64.

I read the #91899 post-mortem before writing this. The job declares RUST_TARGET like every other one and runs rustup target add ${RUST_TARGET}, so the rust-toolchain.toml pin gets its rust-std for aarch64 the same way; it spells the before_script out rather than extending .rust-target-install only because the cross linker has to be apt-installed in the same block. Nothing is added to targets in rust-toolchain.toml, per the note there.

If a rust-linux-aarch64.yml template ever lands, this job collapses to an extends: line.

Tested on real aarch64 hardware

Built and run natively on an aarch64 Linux machine (Snapdragon 7c / SC7180, postmarketOS), target aarch64-unknown-linux-gnu. Worth noting for the pin: rustup honoured rust-toolchain.toml and pulled a fresh 1.96.0 aarch64 toolchain on its own, and the build was clean on it.

• no source changes needed — cargo build --release --target aarch64-unknown-linux-gnu is clean across all the workspace crates (cpu, ppu, apu, mappers, core, libretro)
• librustynes.so, ELF 64-bit LSB shared object, ARM aarch64, 1.9 MB
• in RetroArch: a homebrew NES ROM boots and plays with correct colours, 60.13 fps against the core's 60.0988 target — full speed
• 600 frames, exit code 0, no crash

One aside, unrelated to this PR: the core cannot be driven by a minimal libretro host that calls retro_set_environment before retro_init. rust-libretro 0.3.2 panics there with retro_set_environment: Core has not been initialized yet! (rust-libretro-0.3.2/src/lib.rs:432). RetroArch happens to call them in the other order so it never trips, but a stricter frontend would. That is upstream rust-libretro, not this repo — flagging it only because it cost me a confusing crash while benchmarking.

What I could not test: I built natively on aarch64, whereas this job cross-compiles from x86_64. I also cannot run GitLab pipelines from a GitHub PR, so the job itself has not been executed by CI yet.

Summary by CodeRabbit

• New Features
  • Added automated Linux ARM64 builds for the libretro core.
  • The new build configuration cross-compiles the core for aarch64-unknown-linux-gnu, enabling support for ARM64 environments.

[coderabbitai]

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