GCC supports ARM targets and the macos platform. It took a couple of hours to add support for the new target triple to Rust, so it should take about the same work to add support to it for each of the GCC frontends (10x couple of hours ~ couple of days, maybe a week).

You don't need to implement neither hardware support nor OS support, just combine the two things that are already implemented, and fix a couple of incompatibilities, but not many (some parts of OS support will be x86 only and these would need fixing, but fixing these should be trivial since you can c&p from the arm+iOS code).

Why would it take months to add support for this to GCC? GCC's architecture would need to be really flawed for this to be true, i.e. , requiring you to re-implement a big chunk of hardware support for each OS, or full OS support for each new hardware. Given how many architectures and OSes GCC supports I just can't imagine this being true.

Unless by this work taking months, you mean "not doing anything the first 99% of the days, and doing all the work in the last couple of days". In that case, sure, this can take as long as you want.