No 'check-builtins' target and builtins unit tests not built for runtimes-based compiler-rt build

[asb]

For the following config:

    mkdir -p build/release.max && cd build/release.max
    cmake -G Ninja \
      -DCMAKE_BUILD_TYPE="Release" \
      -DLLVM_ENABLE_PROJECTS="clang;clang-tools-extra;lld;lldb;mlir;polly" \
      -DLLVM_ENABLE_RUNTIMES="compiler-rt;libc;libcxx;libcxxabi;libunwind;openmp;pstl" \
      -DLLVM_ENABLE_ASSERTIONS=ON \
      -DLLVM_CCACHE_BUILD=ON \
      -DCMAKE_C_COMPILER=clang \
      -DCMAKE_CXX_COMPILER=clang++ \
      -DLLVM_ENABLE_LLD=True \
      -DLLVM_TARGETS_TO_BUILD="all" \
      -DLLVM_BUILD_TESTS=True \
      -DLLVM_APPEND_VC_REV=False \
      -DCOMPILER_RT_INCLUDE_TESTS=ON \
      -DCOMPILER_RT_BUILD_SANITIZERS=False \
      ../../llvm
      cmake --build .

ninja check-all and ninja check-compiler-rt or ninja check-runtimes all fail to build / execute the builtins tests in compiler-rt/test/builtins/Unit and there is no check-builtins target. I can see the builtins themselves are built in ./runtimes/builtins-bins/CMakeFiles/clang_rt.builtins-x86_64.dir/. Possible this is user error of course, but seems surprising behaviour regardless. Found via crafting a buld-as-much-as-possible X86 configuration to act as a baseline for some work on another target.

CC @petrhosek