[llvm] [LLVM] Add `LLVM_<proj>_RUNTIME_TARGETS` to set targets per-project (PR #81557)

[Petr Hosek via llvm-commits]

petrhosek wrote:

> Right now the suggested method would be.
> 
> ```
> -DLLVM_ENABLE_RUNTIMES="libc;libc++;openmp"
> -DLLVM_RUNTIMES_TARGETS="default;amdgcn-amd-amdhsa"
> -DRUNTIMES_amdgcn-amd-amdhsa_ENABLE_RUNTIMES='libc'
> ```
> 
> This would have the desired behavior of making the `libc` project compile under the AMDGPU triple. But it would still make it present under the `default` target, which is not the desired behavior.

If you don't want `libc` being present in the `default` target, why not simply omit it from `LLVM_ENABLE_RUNTIMES`?

```
-DLLVM_ENABLE_RUNTIMES="libc++;openmp"
-DLLVM_RUNTIMES_TARGETS="default;amdgcn-amd-amdhsa"
-DRUNTIMES_amdgcn-amd-amdhsa_ENABLE_RUNTIMES="libc"
```

https://github.com/llvm/llvm-project/pull/81557