> I'm somewhat afraid of us drumming up hype for this feature only to be left holding the bag if, six months from now, only x86 and x64 are supported with no ARM patches on the horizon

It doesn't matter how long it takes for the feature to be finished in LLVM because it will be behind a feature gate until it's solid enough for Rust.

> Having no idea what the development process for LLVM is like, is there any sort of concrete commitment to supporting this feature on less-popular architectures?

The patch adding x86 / x86_64 support changed 95 lines. It's not like it would a substantial task to implement this for other architectures. It's part of the portable language specification so any target not implementing it can be considered incomplete and not supported upstream. You can consider that a commitment to implementing it.