[llvm] Improve compiler-rt build structure and export compilation info (PR #190088)

[Chandler Carruth via llvm-commits]

chandlerc wrote:

I can look into symbolic macros, although I'm largely trying to factor into rules as early as I can.

> But I digress :D. LGTM modulo some naming scheme that doesn't suggest that these exclusively reduce inputs ❤️

Well, it _does_ exclusively reduce the inputs. ;] The inputs contain both the generic file _and_ the arch-specific file. Happy to use better names here, mostly not sure what the better name is...

https://github.com/llvm/llvm-project/pull/190088