Fix ARM linker
By default, `cc` is used for the final linking of servo, which points to the host's compiler. This patch ensures that the correct linker is used for cross-compiling to ARM.
Part of #6327
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Upgrade to Spidermonkey 39
> Here it is.
>
> ~~There's two major things that are unfinished here:~~
> - ~~Dealing with the unroot_must_root lint. I'm not sure about the value of this lint with the new rooting API.~~ Done.
> - ~~Updating the Cargo.locks to point to the new SM and SM binding.~~ Done.
>
> I also included my fixes for the rust update, but these will disappear in a rebase. A rust update is necessary to support calling `Drop` on `Heap<T>` correctly when `Heap<T>` is inside a `Rc<T>`. Otherwise `&self` points to the wrong location.
>
> Incremental GC is disabled here. I'm not sure how to deal with the incremental barriers so that's left for later.
>
> Generational GC works. SM doesn't work without it.
>
> The biggest change here is to the rooting API. `Root` was made movable, and `Temporary` and `JSRef` was removed. Movable `Root`s means there's no need for `Temporary`, and `JSRef`s aren't needed generally since it can be assumed that being able to obtain a reference to a dom object means it's already rooted. References have their lifetime bound to the Roots that provided them. DOM objects that haven't passed through `reflect_dom_object` don't need to be rooted, and DOM objects that have passed through `reflect_dom_object` can't be obtained without being rooted through `native_from_reflector_jsmanaged` or `JS::<T>::root()`.
>
> Support for `Heap<T>` ended up messier than I expected. It's split into two commits, but only because it's a bit difficult to fold them together. Supporting `Heap<T>` properly requires that that `Heap::<T>::set()` be called on something that won't move. I removed the Copy and Clone trait from `Heap<T>` so `Cell` can't hold `Heap<T>` - only `UnsafeCell` can hold it.
>
> `CallbackObject` is a bit tricky - I moved all callbacks into `Rc<T>` in order to make sure that the pointer inside to a `*mut JSObject` doesn't move. This is necessary for supporting `Heap<T>`.
>
> `RootedCollectionSet` is very general purpose now. Anything with `JSTraceable` can be rooted by `RootedCollectionSet`/`RootedTraceable`. Right now, `RootedTraceable` is only used to hold down dom objects before they're fully attached to their reflector. I had to make a custom mechanism to dispatch the trace call - couldn't figure out how to get trait objects working for this case.
>
> This has been tested with the following zeal settings:
>
> GC after every allocation
> JS_GC_ZEAL=2,1
>
> GC after every 100 allocations (important for catching use-after-free bugs)
> JS_GC_ZEAL=2,100
>
> Verify pre barriers
> JS_GC_ZEAL=4,1
>
> Verify post barriers
> JS_GC_ZEAL=11,1
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Don't generate pyc files.
This eliminates the last bit of script crate generating in-tree
files. This now allows cargo target dir sharing to fully work.
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This is necessary to ensure Cargo knows when to rebuild. Normally
.gitignore would be enough to exclude these from Cargo's freshness
calculation, but https://github.com/rust-lang/cargo/issues/1729 prevents
this currently. This is the new, correct way to do these thigns, just
like the style crate does.
Make an early return when the WebSocket connection fails in the constructor.
Also let the WebSocket connection to be closed when the connection could
not be established.
Fixes#6082.
