[llvm] [Runtimes] Rework and remove default runtimes build (PR #158156)

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> > Ping, I'm hoping this change is NFC in practice. Also I forgot to mention that this solves #98897.
> 
> Unfortunately it's not, this change would break builds on Apple platforms because they don't use target triple for building runtimes, that's the reason why the `default` target was originally introduced. I have an idea for addressing this but that's going to require more significant changes to the build and Clang.

Interesting, could you elaborate on this? As far as I understand, each target has a default target triple. The main difference here is the directory naming and the fact that it passes `--target=` to the internal CMake job. Is this the part that causes issues? Whatever solution we end up with it will be very important that users can use `default` or similar to target the user's default environment, because we'd want to provide build scripts and cache files that don't require users to fully understand the concept of an LLVM triple.

https://github.com/llvm/llvm-project/pull/158156