[ARM][MC] Add GNU Alias for ldrexd, ldaexd, stlexd, and strexd instructions (#86507)

These aliases were supported previously there was a regression at some point.

This adds back the alternate forms and tidies up this section of code a little.

See https://github.com/llvm/llvm-project/pull/83436#issuecomment-2010213714 for the initial report regarding this change.