Add support for Linux aarch64 builds

images/base-linuxarm64/Dockerfile

@@ -0,0 +1,57 @@
+ARG GH_REPO=btbn/ffmpeg-builds
+FROM ghcr.io/$GH_REPO/base:latest
+
+RUN --mount=src=ct-ng-config,dst=/.config \
+    git clone --filter=blob:none https://github.com/crosstool-ng/crosstool-ng.git /ct-ng && cd /ct-ng && \
+    ./bootstrap && \
+    ./configure --enable-local && \
+    make -j$(nproc) && \
+    cp /.config .config && \
+    ./ct-ng build && \
+    cd / && \
+    rm -rf ct-ng
+
+# Prepare cross environment to heavily favour static builds
+RUN \
+    find /opt/ct-ng -type l \
+        -and -name '*.so' \
+        -and -not -ipath '*plugin*' \
+        -and -not -name 'libdl.*' \
+        -and -not -name 'libc.*' \
+        -and -not -name 'libm.*' \
+        -and -not -name 'libmvec.*' \
+        -and -not -name 'librt.*' \
+        -and -not -name 'libpthread.*' \
+        -delete && \
+    find /opt/ct-ng \
+        -name 'libdl.a' \
+        -or -name 'libc.a' \
+        -or -name 'libm.a' \
+        -or -name 'libmvec.a' \
+        -or -name 'librt.a' \
+        -or -name 'libpthread.a' \
+        -delete && \
+    mkdir /opt/ffbuild
+
+RUN rustup target add aarch64-unknown-linux-gnu
+
+ADD toolchain.cmake /toolchain.cmake
+ADD cross.meson /cross.meson
+
+ADD gen-implib.sh /usr/bin/gen-implib
+RUN git clone --filter=blob:none --depth=1 https://github.com/yugr/Implib.so /opt/implib
+
+ENV PATH="/opt/ct-ng/bin:${PATH}" \
+    FFBUILD_TARGET_FLAGS="--pkg-config=pkg-config --cross-prefix=aarch64-ffbuild-linux-gnu- --arch=aarch64 --target-os=linux" \
+    FFBUILD_TOOLCHAIN=aarch64-ffbuild-linux-gnu \
+    FFBUILD_CROSS_PREFIX="aarch64-ffbuild-linux-gnu-" \
+    FFBUILD_RUST_TARGET="aarch64-unknown-linux-gnu" \
+    FFBUILD_PREFIX=/opt/ffbuild \
+    FFBUILD_CMAKE_TOOLCHAIN=/toolchain.cmake \
+    PKG_CONFIG=pkg-config \
+    PKG_CONFIG_LIBDIR=/opt/ffbuild/lib/pkgconfig:/opt/ffbuild/share/pkgconfig \
+    CFLAGS="-static-libgcc -static-libstdc++ -I/opt/ffbuild/include -O2 -pipe -fPIC -DPIC -D_FORTIFY_SOURCE=2 -fstack-protector-strong -pthread" \
+    CXXFLAGS="-static-libgcc -static-libstdc++ -I/opt/ffbuild/include -O2 -pipe -fPIC -DPIC -D_FORTIFY_SOURCE=2 -fstack-protector-strong -pthread" \
+    LDFLAGS="-static-libgcc -static-libstdc++ -L/opt/ffbuild/lib -O2 -pipe -fstack-protector-strong -Wl,-z,relro,-z,now -pthread -lm" \
+    STAGE_CFLAGS="-fvisibility=hidden -fno-semantic-interposition" \
+    STAGE_CXXFLAGS="-fvisibility=hidden -fno-semantic-interposition"