feat: llvm compiler-rt via osxcross

it adds llvm compiler-rt https://compiler-rt.llvm.org/
compiler-rt can be needed to build code using `__builtin_available()`

it fixes the following error on latest rust-linux-darwin-builder:1.60.0 or earlier

docker run --rm -it -v $PWD:/root/src \
    joseluisq/rust-linux-darwin-builder:1.60.0 \
    sh -c "x86_64-apple-darwin21.1-clang -mmacosx-version-min=10.12 main.c"

Undefined symbols for architecture x86_64:
  "___isOSVersionAtLeast", referenced from:
      _main in main-9c241f.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

docker/Dockerfile

@@ -40,6 +40,8 @@ ARG OSX_CROSS_COMMIT=be2b79f444aa0b43b8695a4fb7b920bf49ecc01c
 RUN set -eux \
     && DEBIAN_FRONTEND=noninteractive apt-get update -qq \
     && DEBIAN_FRONTEND=noninteractive apt-get install -qq -y --no-install-recommends --no-install-suggests \
+        autoconf \
+        automake \
         build-essential \
         ca-certificates \
         clang \
@@ -48,21 +50,27 @@ RUN set -eux \
         file \
         gcc-arm-linux-gnueabihf \
         git \
+        libbz2-dev \
         libgmp-dev \
+        libicu-dev \
         libmpc-dev \
         libmpfr-dev \
         libpq-dev \
         libsqlite3-dev \
         libssl-dev \
+        libtool \
         libxml2-dev \
         linux-libc-dev \
+        llvm-dev \
         lzma-dev \
         musl-dev \
         musl-tools \
         patch \
         pkgconf \
         python \
+        python3 \
         xutils-dev \
+        yasm \
         zlib1g-dev \
     # Clean up local repository of retrieved packages and remove the package lists
     && apt-get clean \
@@ -188,12 +196,29 @@ RUN set -eux \
     && echo "${OSX_SDK_SUM}  ./tarballs/MacOSX${OSX_SDK_VERSION}.sdk.tar.xz" \
         | sha256sum -c - \
     && env UNATTENDED=yes OSX_VERSION_MIN=${OSX_VERSION_MIN} ./build.sh \
-    && rm -rf *~ taballs *.tar.xz \
-    && rm -rf /tmp/* \
+    && true
+
+RUN set -eux \
+    && cd /usr/local/osxcross \
+    && echo "Building osxcross with compiler-rt..." \
+    # compiler-rt can be needed to build code using `__builtin_available()`
+    && env DISABLE_PARALLEL_ARCH_BUILD=1 ./build_compiler_rt.sh \
     && true
 
 ENV PATH $PATH:/usr/local/osxcross/target/bin
 
+RUN set -eux \
+    && echo "Testing osxcross with compiler-rt..." \
+    && echo "int main(void){return 0;}" | xcrun clang -xc -o/dev/null -v - 2>&1 | grep "libclang_rt" 1>/dev/null \
+    && echo "compiler-rt installed and working successfully!" \
+    && true
+
+RUN set -eux \
+    && echo "Removing osxcross temp files..." \
+    && rm -rf *~ taballs *.tar.xz \
+    && rm -rf /tmp/* \
+    && true
+
 WORKDIR /root/src
 
 CMD ["bash"]