ci: add a Linux aarch64 build

.gitlab-ci.yml

@@ -116,6 +116,29 @@ libretro-build-linux-x64:
   variables:
     RUST_TARGET: x86_64-unknown-linux-gnu
 
+# Linux aarch64
+# Cross-compiled from the same image as the x64 job, the way that image already
+# cross-compiles the Apple and Android arm64 targets. There is no
+# rust-linux-aarch64.yml template yet (libretro/libretro-super#2030) and the
+# rust image is amd64-only, so the target and its linker are added here.
+#
+# `before_script` is spelled out instead of extending `.rust-target-install`
+# because the cross linker has to be installed as well; the `rustup target add
+# ${RUST_TARGET}` line is the same one that helper provides, so the #91899
+# toolchain-pin trap is handled identically.
+libretro-build-linux-aarch64:
+  extends:
+    - .libretro-rust-linux-x64
+    - .core-defs
+  variables:
+    RUST_TARGET: aarch64-unknown-linux-gnu
+    TARGET_ARCH: aarch64-unknown-linux-gnu
+    STRIP: aarch64-linux-gnu-strip
+    CARGO_TARGET_AARCH64_UNKNOWN_LINUX_GNU_LINKER: aarch64-linux-gnu-gcc
+  before_script:
+    - apt-get update -qy && apt-get install -yq gcc-aarch64-linux-gnu
+    - rustup target add ${RUST_TARGET}
+
 # macOS 64-bit
 libretro-build-osx-x64:
   extends: