Support RISC-V?

[SUPERCILEX]

A compilation error was introduced in #301 for riscv. Can it be supported natively?

For searchability:

error: The prebuilt `sys.rs` may not be compatible with your target,
       please use bindgen feature to generate new `sys.rs` of your arch
       or use `--cfg=io_uring_skip_arch_check` to skip the check.

[quininer]

Sure, but I don't have a riscv device to test with, and if it's guaranteed on CI (see https://github.com/tokio-rs/io-uring/blob/master/.github/workflows/ci.yml#L36), I'd be happy to take it.

[quininer]

Anyway, now you can skip check using io_uring_skip_arch_check, or specify your own binding using io_uring_use_own_sys