Support for HermitOS

[notgull]

hermit_abi is the official interface to HermitOS. It kind of lines up with libc, so it might be possible to build it into the libc backend.

cc smol-rs/polling#177
cc @stlankes

[xtqqczze]

This issue likely depends on bumping MSRV to 1.66 for std::os::fd support on HermitOS.

No, I think it's fine require a newer Rust toolchain for Tier 3 targets. There are nightly only in CI anyway.

[xtqqczze]

Blocked on rust-lang/libc#5460 for no_std support.