This patches paves way for upcoming SVE RegisterInfo definitions. This is cosmetic change which allows us to define ARM64 RegisterInfo using macros.
In future we ll have define two different RegisterInfos to choose between SVE vs non-SVE RegisterInfo with decision being made at thread creation.
Since most definitions don't have an alt, might be nice to make two:
Also be nice to not assume "alt" is already a string and just add the #alt into the definition. See comment below for more details