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

[Jonathan Thackray via llvm-commits]

https://github.com/jthackray approved this pull request.

You've added new test cases for the (undocumented) GNU shortforms.  Code looks good to me. Approved.

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