fs::set_permissions/chmod unsupported on Linux

[barely-a-dev]

Hello, calling fs::set_permissions(path, Permissions::from_mode(0o700))?; fails with the below message.

--> ~/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/sys/pal/unix/fs.rs:1815:50
     |
1815 |     run_path_with_cstr(p, &|p| cvt_r(|| unsafe { libc::chmod(p.as_ptr(), perm.mode) }).map(|_| ()))
     |                                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't call foreign function `chmod` on OS `linux`
...
 help: if this is a basic API commonly used on this target, please report an issue with Miri

Since I'm thinking that chmod is probably a pretty basic API commonly used on Linux, I made this issue.

[RalfJung]

Permissions::from_mode is unix-only, so we'll not be able to easily support this on Windows hosts unfortunately, so the tests will have to use ignore-host. But other than that, shouldn't be very hard to add.