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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