ci: add Linux aarch64 libretro build

[WizzardSK]

The buildbot publishes mupen64plus_next_libretro.so for linux/x86_64 and linux/x86, but there is no desktop Linux ARM64 build — .gitlab-ci.yml has no aarch64 job, so nothing is produced for that platform, even though the core already carries an aarch64 dynarec (NEW_DYNAREC=4, new_dynarec/arm64/linkage_arm64.S).

This adds the /linux-aarch64.yml template include and a libretro-build-linux-aarch64 job carrying the same variables as the existing x64 one (HAVE_PARALLEL_RDP, HAVE_PARALLEL_RSP, HAVE_THR_AL, LLE). No image or compiler override is needed: the libretro-build-aarch64-ubuntu image installs gcc-12/g++-12 and points gcc/g++ at them through update-alternatives.

Verified on real aarch64 hardware, and not only as a build:

• Builds clean from this branch with those variables set.
• The dynarec is the one that gets used: the core logs Starting R4300 emulator: Dynamic Recompiler / Init new dynarec, not the interpreter.
• Mario Kart 64 runs in RetroArch on aarch64 — through the boot sequence and intro into the MAP SELECT menu, with the cups, the track preview and the track list all rendering correctly through GLideN64 (RetroArch negotiated a glcore 3.3 context). No errors in the RetroArch log.

Two notes for whoever reviews this, since neither is caused by the change but both could bite someone reproducing it:

• ParaLLEl-RDP could not be exercised on this particular device. The GPU here is an Adreno 618 on Mesa/Turnip, which does not expose VK_KHR_16bit_storage for SSBOs, and paraLLEl-RDP refuses to start without it. That is a driver limitation on my hardware, not an ARM64 problem — the job still builds paraLLEl-RDP, and the GLideN64 path is verified above.
• parallel-rdp/util/aligned_alloc.cpp needs #include <cstdint> on GCC 13+, which no longer pulls it in transitively; it fails there with 'uintptr_t' was not declared in this scope. It is unrelated to architecture and does not affect the CI image (gcc-12), so it is deliberately not part of this PR — but it will surface for anyone building with a newer toolchain, and belongs upstream in parallel-rdp.

Only the Linux ARM64 job is added; nothing else in the file changes.