After JDK-8253255, we no longer use ABI attributes to do classification, but instead use a custom CValueLayout subtype. This type also overrides describeConstable to return a constant descriptor that, upon resolution, will load the corresponding field in CLinker, instead of trying to create a ValueLayout from scratch.

This means we can remove the jextract code that currently manually tries to do the same, which is unfortunately having to duplicate a lot of code from the layout classes.