Drop custom makedev implementation for Redox

[jackpot51]

These functions were added to libc 0.2.181 in rust-lang/libc#4963.

[xtqqczze]

@sunfishcode Can we get this merged and a new rustix release ASAP, to fix compile errors:

error[E0308]: mismatched types
  --> src/backend/libc/fs/makedev.rs:53:5
   |
50 |   pub(crate) fn makedev(maj: u32, min: u32) -> Dev {
   |                                                --- expected `u64` because of return type
...
53 | /     ((i64::from(maj) & 0xffff_f000_i64) << 32)
54 | |         | ((i64::from(maj) & 0x0000_0fff_i64) << 8)
55 | |         | ((i64::from(min) & 0xffff_ff00_i64) << 12)
56 | |         | (i64::from(min) & 0x0000_00ff_i64)
   | |____________________________________________^ expected `u64`, found `i64`
   |
help: you can convert an `i64` to a `u64` and panic if the converted value doesn't fit
   |
53 ~     (((i64::from(maj) & 0xffff_f000_i64) << 32)
54 |         | ((i64::from(maj) & 0x0000_0fff_i64) << 8)
55 |         | ((i64::from(min) & 0xffff_ff00_i64) << 12)
56 ~         | (i64::from(min) & 0x0000_00ff_i64)).try_into().unwrap()
   |

For more information about this error, try `rustc --explain E0308`.
error: could not compile `rustix` (lib) due to 1 previous error

[jackpot51]

@sunfishcode could you please take a look?

[xtqqczze]

Blocked by #1584