[PATCH][AArch64]Fix the Cygwin build failure caused by questionable enum values in AArch64BaseInfo.h

[Tim Northover]

> I don’t have the patch to look at. It sounds like a refactoring patch, though. I’m fine with it going in if it helps and doesn’t change functionality.

Yep. It's just "rename a bunch of enumerators so that they don't start
with "_[A-Z]". Thanks Bill.

Could you do the merge onto 3.4 Hao?

Thanks

Tim.