[Morello] Add the morello descriptor ABI specification.

[sbaranga-arm]

[jrtc27]

How do you handle function pointer equality in the presence of dlsym, or even just multiple dynamic relocations for the same descriptor? What actually goes in .desc.data.rel.ro?

[sbaranga-arm]

At the moment it's the job of the dynamic linker to make sure only one pair of capabilities is allocated per function pointer while resolving the relocations, and that provides function pointer equality. There's likely scope for improvement here though.