[clang] [openmp] [OpenMP] Respect LLVM per-target install directories (PR #83282)

[Ye Luo via cfe-commits]

ye-luo wrote:

I'm OK with the first two
```
-DLLVM_RUNTIME_TARGETS=default;amdgcn-amd-amdhsa;nvptx64-nvidia-cuda
Enables the runtimes for the target triples, default is what you get without specifying anything
```
I actually think default should automatically include amdgcn and nvptx when I have amdgcn and nvptx backend turned on.

```
-DLLVM_ENABLE_RUNTIMES="openmp"
This specifies the default target builds the openmp runtime.
```

Then openmp cmake can react to the contents of `LLVM_RUNTIME_TARGETS` by adding device runtime for `x86,amdgcn,nvptx`

https://github.com/llvm/llvm-project/pull/83282