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

[Petr Hosek via llvm-commits]

petrhosek 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.

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