LLVM_RUNTIMES_TARGET gets ignored while building libclc out-of-tree

I'm building libclc with the following command:

```sh
CC=$(llvm-config --bindir)/clang cmake libclc/CMakeLists.txt -G Ninja -DCMAKE_BUILD_TYPE=Release -DLLVM_DIR="$(llvm-config --cmakedir)" -DLLVM_RUNTIMES_TARGET=amdgcn-amd-amdhsa-llvm
```

but it ignores the option and builds for all runtimes present in the target llvm build anyways, and prints this warning:

```
CMake Warning:
Manually-specified variables were not used by the project:

LLVM_RUNTIMES_TARGET
```

Start with libclc/CMakeLists.txt and reproduce the out-of-tree Ninja configuration using LLVM_RUNTIMES_TARGET. Trace where the option is expected to be consumed, then verify CMake no longer warns that it is unused and that the build targets only amdgcn-amd-amdhsa-llvm rather than every available runtime.

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