Update compiler-rt with clang?

See

https://github.com/emscripten-core/emscripten/pull/24160#issuecomment-4846400577

By leaving compiler-rt on older versions, we risk breakage like with fcoverage. So if we want to support those features, it seems we don't have a choice but to update compiler-rt to tip of tree constantly.

Is that a lot of work, though?

Start by reviewing the linked Emscripten pull request and its referenced comment; the issue names no files, tests, or update entry point. Determine how compiler-rt is updated alongside clang, with completion defined as a clear approach that avoids the older-version breakage described.

Written by the indexing model from the issue text.