build-openmp: Switch to building through the runtimes directory

Since 6295b8e763503644961f9087382f80965b98d466 in llvm-project,
it's no longer possible to build openmp entirely standalone;
now one has to build it through the runtimes directory.

build-openmp.sh

@@ -48,7 +48,7 @@ if [ ! -d llvm-project/openmp ] || [ -n "$SYNC" ]; then
     CHECKOUT_ONLY=1 ./build-llvm.sh
 fi
 
-cd llvm-project/openmp
+cd llvm-project/runtimes
 
 if command -v ninja >/dev/null; then
     CMAKE_GENERATOR="Ninja"
@@ -76,9 +76,9 @@ for arch in $ARCHS; do
         ;;
     esac
 
-    [ -z "$CLEAN" ] || rm -rf build-$arch
-    mkdir -p build-$arch
-    cd build-$arch
+    [ -z "$CLEAN" ] || rm -rf build-openmp-$arch
+    mkdir -p build-openmp-$arch
+    cd build-openmp-$arch
     [ -n "$NO_RECONF" ] || rm -rf CMake*
 
     cmake \
@@ -92,6 +92,7 @@ for arch in $ARCHS; do
         -DCMAKE_SYSTEM_NAME=Windows \
         -DCMAKE_AR="$PREFIX/bin/llvm-ar" \
         -DCMAKE_RANLIB="$PREFIX/bin/llvm-ranlib" \
+        -DLLVM_ENABLE_RUNTIMES="openmp" \
         -DLIBOMP_ENABLE_SHARED=TRUE \
         -DCMAKE_C_FLAGS_INIT="$CFGUARD_CFLAGS" \
         -DCMAKE_CXX_FLAGS_INIT="$CFGUARD_CFLAGS" \