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

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> Oh, that's `llvm/runtimes` and not top-level `runtimes`, so I guess it won't affect Gentoo.

Yes, ideally this change doesn't affect anyone that isn't depending on the name of directories in the build tree. Fundamentally I'm hoping to make our runtimes infrastructure more predictable by removing this weird disconnect we have between the `generic` and specific targets. More or less, this change is equivalent to the user putting `-DRUNTIME_TARGETS=${LLVM_DEFAULT_TARGET_TRIPLE}`. I want to keep the functionality of `default` because it's very important  that things like cache files can target the user's default system.

I added the `check` targets to give an interface that looks the same as what we have now for the majority of users.

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