Auto merge of #18189 - JJayet:ios_support, r=larsbergstrom

First step toward iOS Support

This mostly prevents Gaol/sandboxing to be used during compilation.
There are a number of issues with : `expat-sys`, `servo-freetype-sys`, `fontsan`, `heartbeats-simple-sys`, `mozjs_sys`.

Also modified `components/script/dom/navigatorinfo.rs` in order to add iOS.

```rust
#[cfg(target_os = "ios")]
pub fn Platform() -> DOMString {
    DOMString::from("iOS")
}
```

See here for more informations : https://github.com/servo/servo/issues/18154

---

- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [ ] These changes partially work on #18154

- [X] These changes do not require tests because it deals with the compile toolchain

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/18189)
<!-- Reviewable:end -->

components/constellation/pipeline.rs

@@ -537,7 +537,7 @@ impl UnprivilegedPipelineContent {
         }
     }
 
-    #[cfg(not(target_os = "windows"))]
+    #[cfg(all(not(target_os = "windows"), not(target_os = "ios")))]
     pub fn spawn_multiprocess(self) -> Result<(), Error> {
         use gaol::sandbox::{self, Sandbox, SandboxMethods};
         use ipc_channel::ipc::IpcOneShotServer;
@@ -585,9 +585,9 @@ impl UnprivilegedPipelineContent {
         Ok(())
     }
 
-    #[cfg(target_os = "windows")]
+    #[cfg(any(target_os = "windows", target_os = "ios"))]
     pub fn spawn_multiprocess(self) -> Result<(), Error> {
-        error!("Multiprocess is not supported on Windows.");
+        error!("Multiprocess is not supported on Windows or iOS.");
         process::exit(1);
     }