Auto merge of #16833 - MortimerGoro:layout_alignment, r=SimonSapin

Fix unsafe AtomicRefCell<T>  transmutes in Layout component

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Fixes unsafe transmute between `AtomicRefCell<PersistentLayoutData>` and `AtomicRefCell<PartialPersistentLayoutData>` which have different memory alignment in 32 bit archs leading to SEGV crashes. See https://github.com/servo/servo/issues/16817 and https://github.com/servo/servo/pull/16816

mem::align_of values in 32 bit archs (e.g. Android):
```
PersistentLayoutData 8
PersistentLayoutData 4
AtomicRefCell<PersistentLayoutData> 8
AtomicRefCell<PartialPersistentLayoutData> 4
```
mem::align_of values in 64 bit archs
```
PersistentLayoutData 8
PersistentLayoutData 8
AtomicRefCell<PersistentLayoutData> 8
AtomicRefCell<PartialPersistentLayoutData> 8
```

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- [x] These changes fix #16817 (github issue number if applicable).

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