[AArch64] Factor out handling of AUTRELLOADPAC addend in AsmPrinter (NFC) (#192076)
In `AArch64AsmPrinter`, move the emission of code that handles the
Addend operand of AUTRELLOADPAC from `emitPtrauthAuthResign` to a
separate function.
Make use of recently added helper functions and update the comments.
Refer to named `Pointer` and `Scratch` registers instead of
hardcoding X16 and X17.