Build docker images for arm64 as well as x86.

This involves:

* A small change to the Dockerfile, so that it checks the target
  architecture and only installs gcc-multilib on x86_64.
* Updates to the GitHub Actions workflow build_aflplusplus_docker.yaml
  to build for both linux/amd64 and linux/arm64.

These changes should allow Mac M1 users to get a working version of
AFL++ by doing "docker pull aflplusplus/aflplusplus".

.github/workflows/build_aflplusplus_docker.yaml

@@ -12,6 +12,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@master
+    - name: Set up QEMU
+      uses: docker/setup-qemu-action@v1
+    - name: Set up Docker Buildx
+      uses: docker/setup-buildx-action@v1
     - name: Login to Dockerhub
       uses: docker/login-action@v1
       with:
@@ -21,5 +25,6 @@ jobs:
       uses: docker/build-push-action@v2
       with:
         context: .
+        platforms: linux/amd64,linux/arm64
         push: true
         tags: aflplusplus/aflplusplus:latest

Dockerfile

@@ -39,13 +39,20 @@ RUN echo "deb http://ppa.launchpad.net/ubuntu-toolchain-r/test/ubuntu focal main
 
 RUN apt-get update && apt-get full-upgrade -y && \
     apt-get -y install --no-install-suggests --no-install-recommends \
-    gcc-10 g++-10 gcc-10-plugin-dev gcc-10-multilib gcc-multilib gdb lcov \
+    gcc-10 g++-10 gcc-10-plugin-dev gdb lcov \
     clang-12 clang-tools-12 libc++1-12 libc++-12-dev \
     libc++abi1-12 libc++abi-12-dev libclang1-12 libclang-12-dev \
     libclang-common-12-dev libclang-cpp12 libclang-cpp12-dev liblld-12 \
     liblld-12-dev liblldb-12 liblldb-12-dev libllvm12 libomp-12-dev \
-    libomp5-12 lld-12 lldb-12 llvm-12 llvm-12-dev llvm-12-runtime llvm-12-tools \
+    libomp5-12 lld-12 lldb-12 llvm-12 llvm-12-dev llvm-12-runtime llvm-12-tools
-    && rm -rf /var/lib/apt/lists/*
+
+# arm64 doesn't have gcc-multilib, and it's only used for -m32 support on x86
+ARG TARGETPLATFORM
+RUN [ "$TARGETPLATFORM" = "linux/amd64" ] && \
+    apt-get -y install --no-install-suggests --no-install-recommends \
+    gcc-10-multilib gcc-multilib || true
+
+RUN rm -rf /var/lib/apt/lists/*
 
 RUN update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-10 0
 RUN update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-10 0