Build the OpenMP runtime in docker based builds

LLVM's OpenMP implementation only supports Windows on x86, and it
requires a MASM-compatible assembler to build it. Build UASM
for this purpose, for now. (UASM isn't available packaged in
debian/ubuntu repos.) In the future, llvm-ml should be able
to handle it instead, but it's not currently mature enough.

Not including it in the regular build-all.sh for now, as UASM
isn't generally available.

Dockerfile

@@ -19,8 +19,18 @@ ARG TOOLCHAIN_ARCHS="i686 x86_64 armv7 aarch64"
 
 ARG DEFAULT_CRT=ucrt
 
+# Build UASM, for building openmp. In the future, llvm-ml should be able
+# to handle it, but it doesn't yet.
+RUN git clone https://github.com/Terraspace/UASM && \
+    cd UASM && \
+    git checkout 16a853bd6de807fe2c42569f8375a029684c0f22 && \
+    make -f gccLinux64.mak -j$(nproc) && \
+    mkdir -p $TOOLCHAIN_PREFIX/bin && \
+    cp GccUnixR/uasm $TOOLCHAIN_PREFIX/bin
+COPY wrappers/uasm-wrapper.sh $TOOLCHAIN_PREFIX/bin
+
 # Build everything that uses the llvm monorepo. We need to build the mingw runtime before the compiler-rt/libunwind/libcxxabi/libcxx runtimes.
-COPY build-llvm.sh strip-llvm.sh install-wrappers.sh build-mingw-w64.sh build-mingw-w64-tools.sh build-compiler-rt.sh build-mingw-w64-libraries.sh build-libcxx.sh ./
+COPY build-llvm.sh strip-llvm.sh install-wrappers.sh build-mingw-w64.sh build-mingw-w64-tools.sh build-compiler-rt.sh build-mingw-w64-libraries.sh build-libcxx.sh build-openmp.sh ./
 COPY wrappers/*.sh wrappers/*.c wrappers/*.h ./wrappers/
 RUN ./build-llvm.sh $TOOLCHAIN_PREFIX && \
     ./strip-llvm.sh $TOOLCHAIN_PREFIX && \
@@ -31,6 +41,7 @@ RUN ./build-llvm.sh $TOOLCHAIN_PREFIX && \
     ./build-mingw-w64-libraries.sh $TOOLCHAIN_PREFIX && \
     ./build-libcxx.sh $TOOLCHAIN_PREFIX && \
     ./build-compiler-rt.sh $TOOLCHAIN_PREFIX --build-sanitizers && \
+    ./build-openmp.sh $TOOLCHAIN_PREFIX && \
     rm -rf /build/*
 
 # Build libssp