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

[Michał Górny via llvm-commits]

mgorny wrote:

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

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