[libc] [llvm] [LLVM] Add 'LLVM_EXTRA_RUNTIME_TARGETS' to append to default list (PR #82513)

[Joseph Huber via llvm-commits]

jhuber6 wrote:

If this lands the expected way to enable the GPU `libc` would become the following.
```
    -DRUNTIMES_nvptx64-nvidia-cuda_LLVM_ENABLE_RUNTIMES=libc   
    -DRUNTIMES_amdgcn-amd-amdhsa_LLVM_ENABLE_RUNTIMES=libc 
    -DLLVM_EXTRA_RUNTIME_TARGETS=amdgcn-amd-amdhsa;nvptx64-nvidia-cuda
```

https://github.com/llvm/llvm-project/pull/82513