(Since my first reply here somehow got lost, I'm posting this again).
Rust doesn't prevent anyone from building Tier 2 or Tier 3 targets. There is no limitation for "legacy" or "deprecated" targets. Any target can be built and any target can be selected by the Rust compiler for cross-compliation.
So, if you want to mimic the behavior of the Rust compiler, you would obviously not guard any architectures or platforms in configure.ac.
I am objecting this change as it does not reduce code complexity (on the contrary) and only causes more maintenance burden for downstream distributions.