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

[Jon Chesterfield via llvm-commits]

JonChesterfield wrote:

Have you looked at pull/81921? That is a huge decrease in spurious complexity that we can't have without something functionally equivalent to this PR. I don't mind what horrendous cmake invocation is required to build things but I do want 81921, it lets me get rid of some really fragile infra for working with libc.

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