Support std::fs::File::seek

[m-mueller678]

std::sys::pal::hermit::fs::File::seek currently always return EINVAL.

[jounathaen]

For reference: Hermit already supports lseek, but apparently this is currently missing in the Rust stdlib. Here is the respective code:
https://github.com/rust-lang/rust/blob/7b31983d5506e975801963b88b36c504eb2cecd8/library/std/src/sys/pal/hermit/fs.rs#L425

[thaliaarchi]

This is blocked by lseek being added since the last release to hermit-abi. If a new release were published, though, I'd be happy to fix this on the Rust side.