[llvm] [CMake] Correctly handle LLVM_ENABLE_RUNTIMES in targets (PR #69869)

[Jonas Devlieghere via llvm-commits]

JDevlieghere wrote:

Looks like this broke the `cxx` target: https://green.lab.llvm.org/green/view/LLDB/job/as-lldb-cmake/8459/

```
$ cmake /Users/jonas/llvm/llvm-project/llvm \
                                       -G Ninja \
                                       -DCMAKE_BUILD_TYPE='RelWithDebInfo' \
                                       -DLLVM_ENABLE_ASSERTIONS:BOOL=ON \
                                       -DLLVM_ENABLE_PROJECTS='llvm;clang' \
                                       -DLLVM_ENABLE_RUNTIMES='libcxx;libcxxabi'
...
$ ninja cxx
ninja: error: unknown target 'cxx', did you mean 'lli'?
```

https://github.com/llvm/llvm-project/pull/69869