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

[Jonathan Thackray via llvm-commits]

jthackray wrote:

> 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?

Alfie, do you think it's worth adding extra test cases here? Or would they provide limited benefit?

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