Add gitlab ci config

.gitlab-ci.yml

+variables:
+  DOCKER_REV: 205afce5c7ebdb8666b96638f6758fe527f40a7f
+  GIT_DEPTH: "1"
+
+x86_64-linux:
+  tags:
+    - x86_64-linux
+  image: registry.gitlab.haskell.org/ghc/ci-images/x86_64-linux-ubuntu20_04:$DOCKER_REV
+  before_script:
+    - sudo chown ghc:ghc -R .
+    - |
+      mkdir -p /tmp/wasi-sdk
+      curl -f -L --retry 5 https://github.com/WebAssembly/wasi-sdk/releases/download/wasi-sdk-16/wasi-sdk-16.0-linux.tar.gz | tar xz --strip-components=1 -C /tmp/wasi-sdk
+
+      curl -f -L --retry 5 https://wasmtime.dev/install.sh | bash
+      export WASMTIME_HOME="$HOME/.wasmtime"
+      export PATH="$WASMTIME_HOME/bin:/tmp/wasi-sdk/bin:$PATH"
+  script:
+    - |
+      BUILD_CORES=$(curl -f -L --retry 5 https://gitlab.haskell.org/ghc/ghc/-/raw/master/mk/detect-cpu-count.sh | sh)
+      echo "with-compiler: $GHC" >> cabal.project.local
+      cabal update
+      cabal v2-run -j$BUILD_CORES libffi-wasm
+
+      clang -Wall -Wextra -O3 -DNDEBUG -Icbits -c cbits/ffi.c -o cbits/ffi.o
+      clang -Wall -Wextra -O3 -DNDEBUG -Icbits -c cbits/ffi_call.c -o cbits/ffi_call.o
+      clang -Wall -Wextra -O3 -DNDEBUG -Icbits -c cbits/ffi_closure.c -o cbits/ffi_closure.o
+
+      mkdir -p out/libffi-wasm/include
+      cp cbits/*.h out/libffi-wasm/include
+      mkdir -p out/libffi-wasm/lib
+      llvm-ar -r out/libffi-wasm/lib/libffi.a cbits/*.o
+
+      pushd cbits_test
+      CC="clang -Wall -Wextra -O3 -DNDEBUG -I../out/libffi-wasm/include -L../out/libffi-wasm/lib -lffi" ./test.sh
+      popd
+  artifacts:
+    expire_in: 1 year
+    paths:
+      - out