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

[Joseph Huber via llvm-commits]

jhuber6 wrote:

Today I have learned about the joys of `;` versus `|` in CMake.

@petrhosek I somewhat understand the difficulties here. It seems that the existing support tries to do `triples=projects` while this does `projects=triples`. I think both can coexist, and have somewhat different use-cases. Do you have any more qualms about this patch? I tried getting the alternative format you provided to work but it didn't seem to do what I wanted even after patching up `libc`.

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