ci: add a Linux aarch64 build

[WizzardSK]

The libretro buildbot ships no aarch64 Linux build of dosbox-core, so the core can't be installed from RetroArch's Core Downloader on ARM64 Linux machines (ARM Chromebooks, SBCs, ARM laptops). Nothing blocks it: the build already knows this architecture - on an aarch64 host it configures itself with -DC_DYNREC -DC_TARGETCPU="ARMV8LE", i.e. the ARMv8 dynrec backend - and android-arm64-v8a already builds these sources for ARM64.

The job mirrors libretro-build-linux-x64:

# Linux ARM 64-bit
libretro-build-linux-aarch64:
  extends:
    - .libretro-linux-aarch64-make-default
    - .core-defs

plus the matching /linux-aarch64.yml template include.

What I verified on aarch64 (postmarketOS on a Snapdragon 670 Chromebook, glibc): every dosbox-core source file - core, libretro glue, voodoo, fluidsynth/BASSMIDI/MT-32 midi backends - compiles cleanly with no source changes and with the ARMv8 dynrec selected, and the bundled dependencies (SDL 1.2, SDL_net, fluidsynth, munt, ogg, vorbis, opus, opusfile, FLAC, libsndfile) all build for aarch64 too.

What I could not finish, and why it isn't an ARM problem: the final link needs deps_bin/lib/libmpg123.a, and bundled mpg123 doesn't compile under my local GCC 15 - deps/mpg123/src/compat/compat.c:500 assigns a void (*)(void) to a __sighandler_t, which GCC 14+ makes a hard error. The buildbot images use GCC 12, where that's only a warning, so CI should be unaffected. Verifying via the CI job on this PR is the way to confirm.

One unrelated thing I noticed while doing this: with BUNDLED_AUDIO_CODECS=1 (the default), Makefile.libretro:266 always links libmpg123.a and libopusfile.a, but neither is a prerequisite of anything - $(OBJECTS) only depends on $(FLUIDSYNTH) $(MUNT) $(SDLNET), and $(LIBMPG123)/$(OPUSFILE) are reachable only through the phony mpg123/opusfile targets. So a clean checkout built with plain make -f Makefile.libretro platform=unix fails at link on any architecture until those two are built by hand. Not something this PR touches, but you may want to add them to the $(OBJECTS) prerequisite line.