This is a non-issue. YJIT only targets x86-64. After all, this is a JIT. If you designed a new architecture X, you need to port YJIT itself to target X, in addition to GCC, LLVM, etc.
Oh, so YJIT is highly coupled to x86-64? Porting GCC + yjit is less work than porting GCC + yjit + LLVM.
It's not like new architectures appear very quickly, much less adopted very quickly. The benefits of maintenance overhead reduction and development speed increase, far outweight the theoretical downside of having to port LLVM to that new architecture.
It’s not that it’s highly coupled, just that it’s still the early days and only x86_64 was on the roadmap. Arm64 is planned, and will hopefully make it into Ruby 3.2
And with an Arm64 backend, adding RISC-V is probably going to be a walk in the park.