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

[Joseph Huber via llvm-commits]

jhuber6 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"
> ```

This is a little broken right now with the current logic, since we use things like 
```if ("libc" IN_LIST LLVM_ENALBE_RUNTIMES")```
in various places. That could possible be fixed if we check through all the runtime targets then.

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