Fix: build target wasm32-unknown-emscripten

[ChanTsune]

What does this PR do

Fix failed to build for target wasm32-unknown-emscripten with this error:

error[E0432]: unresolved import `self::consts`
  --> src/errno.rs:19:15
   |
19 | pub use self::consts::*;
   |               ^^^^^^ could not find `consts` in `self`

error[E0432]: unresolved import `self::Errno`
   --> src/errno.rs:198:15
    |
198 |     use self::Errno::*;
    |               ^^^^^ could not find `Errno` in `self`

error[E0432]: unresolved import `crate::errno::Errno`
 --> src/fcntl.rs:2:5
  |
2 | use crate::errno::Errno;
  |     ^^^^^^^^^^^^^^-----
  |     |             |
  |     |             help: a similar name exists in the module: `errno`
  |     no `Errno` in `errno`

error[E0432]: unresolved import `crate::errno::Errno`
 --> src/sys/signal.rs:3:5
  |
3 | use crate::errno::Errno;
  |     ^^^^^^^^^^^^^^-----
  |     |             |
  |     |             help: a similar name exists in the module: `errno`
  |     no `Errno` in `errno`

error[E0432]: unresolved import `crate::errno::Errno`
 --> src/unistd.rs:3:5
  |
3 | use crate::errno::Errno;
  |     ^^^^^^^^^^^^^^-----
  |     |             |
  |     |             help: a similar name exists in the module: `errno`
  |     no `Errno` in `errno`

error[E0432]: unresolved import `errno::Errno`
   --> src/lib.rs:205:5
    |
205 | use errno::Errno;
    |     ^^^^^^^-----
    |     |      |
    |     |      help: a similar name exists in the module: `errno`
    |     no `Errno` in `errno`

For more information about this error, try `rustc --explain E0432`.
error: could not compile `nix` (lib) due to 6 previous errors

To pass the build, I made the following changes:

• Added the errno::errno_location function for Emscripten.
• Uses the same errno::consts module as Linux.

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API

[SteveLauC]

CI fixed in #2478, please rebase your branch:)

[SteveLauC]

Thanks for fixing it for Nix, just one tiny issue:)

[SteveLauC]

BTW, perhaps we can have a changelog for this, please see CONTRIBUTING.md on how to add one, something like this would suffice:

2477.added.md

Add the support for Emscripten.

[SteveLauC]

LGTM, thanks!

[SteveLauC]

Ahh, our Cirrus CI has been quite unstable recently...

[SteveLauC]

Damn it, I think our Cirrus CI is broken, which is definitely not related to this PR, I will take a look later:<

[SteveLauC]

Weird, Cirrus CI works fine in other PRs, let's merge this PR anyway!