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

[Petr Hosek via llvm-commits]

petrhosek wrote:

I'm working on a larger refactor that eliminates the `default` as a special target altogether without a need to introduce additional CMake option.

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