The linker has a description about which layouts are supported: https://github.com/openjdk/jdk/blob/e9d19d0fffc47119d0d4f756833ec87cd0a6331e/src/java.base/share/classes/java/lang/foreign/Linker.java#L227-L248

This describes a set of alignment and size constraints, mostly targeted at group layouts.

We want to relax this text to allow linkers to support additional layouts as well. This is useful for instance for the PPC AIX ABI, which commonly supports packed structs.