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

[Petr Hosek via llvm-commits]

petrhosek wrote:

@jhuber6 @JonChesterfield Can you give me a more concrete example of how you intend to build the full set of runtimes (including compiler-rt, libc and libcxx) for GPU using this mechanism? Maybe that would help me better understand the motivation.

The reason I'm asking is because I'm still unconvinced by the simplicity arguments. The reason for pushing this feature is to simplify the runtimes build compared to current implementation that requires multiple variables, but in my experience, building runtimes typically requires a number of variables regardless, so saving one variable for setting the list of runtimes makes it difficult to justify more complexity in the runtimes build for me.

For reference, here's an example from our runtimes build for Linux (which includes compiler-rt, libunwind, libcxxabi and libcxx):

https://github.com/llvm/llvm-project/blob/c098f2d1669fb1ec6a5eac81277bd16b334ec956/clang/cmake/caches/Fuchsia-stage2.cmake#L146-L191

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