github: Try to test openmp through the runtimes directory

Since 6295b8e763503644961f9087382f80965b98d466 in llvm-project,
one no longer can configure the openmp project entirely standalone,
one has to do it through the runtimes directory.

However, in practice, this currently leads to no tests being
executed at all, as the runtimes build setup doesn't support
pointing to an external FileCheck executable - it currently has
to be part of a full bootstrapping build.

.github/workflows/build.yml

@@ -598,7 +598,7 @@ jobs:
           cd llvm-project
           mkdir build-openmp
           cd build-openmp
-          cmake ../openmp `
+          cmake ../runtimes `
             -G Ninja `
             -DCMAKE_BUILD_TYPE=Release `
             -DPython3_EXECUTABLE="$Env:PYTHON_EXE" `
@@ -606,6 +606,7 @@ jobs:
             -DCMAKE_C_COMPILER=clang `
             -DCMAKE_CXX_COMPILER=clang++ `
             -DCMAKE_ASM_MASM_COMPILER=llvm-ml `
+            -DLLVM_ENABLE_RUNTIMES="openmp" `
             -DLIBOMP_ASMFLAGS=${{matrix.asmflag}} `
             -DOPENMP_LIT_ARGS="-v --time-tests"
           ninja