[llvm] [ARM][MC] Add GNU Alias for ldrexd and strexd instructions (PR #86507)

[Alfie Richards via llvm-commits]

AlfieRichardsArm wrote:

> I can confirm that when I try to build our internal code base that was failing earlier with these changes it now successfully builds again.
> 
> I have also looked over your change and it generally looks fine to me, although I would prefer that someone more familiar with this area actually approves it before committing the change.
> 
> Regarding testing, I notice that this change also seems to affect `ldaexd` and probably `stlexd`. Are there tests for these instructions in this case or more generally? Also I'm not sure if we generally do negative tests for these things, but if we do, can we add negative tests for destination/source operands not being sequential and not starting at an even register?

`ldaexd` and `stlexd` is a good point. I will check what GCC's behaviour is and match that. I will also add negative tests

https://github.com/llvm/llvm-project/pull/86507