[ARM]: codegen `llvm.roundeven.v*` (#141786)
fixes https://github.com/llvm/llvm-project/issues/73588
The aarch64 version of `frintn.ll` notes the intention to auto-upgrade
`frintn` to `roundeven`. I haven't been able to figure out how to make
that happen though (either for arm or aarch64).
The original issue came up in
https://github.com/rust-lang/stdarch/pull/1807
