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.dev

@@ -127,6 +127,34 @@ RUN cd test && \
         done; \
     done
 
+# 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 OpenMP
+COPY build-openmp.sh ./
+RUN ./build-openmp.sh $TOOLCHAIN_PREFIX
+
+# OpenMP on windows only supports x86.
+RUN cd test && \
+    for arch in $TOOLCHAIN_ARCHS; do \
+        case $arch in \
+        i686|x86_64) \
+            ;; \
+        *) \
+            continue \
+            ;; \
+        esac && \
+        for test in hello-omp; do \
+            $arch-w64-mingw32-clang $test.c -o $arch/$test.exe -fopenmp=libomp || exit 1; \
+        done; \
+    done
+
 RUN cd test && \
     for arch in $TOOLCHAIN_ARCHS; do \
         cp $TOOLCHAIN_PREFIX/$arch-w64-mingw32/bin/*.dll $arch || exit 1; \