[AArch64] Match the windows canonical callee saved register order

On windows, the callee saved registers in a canonical prologue are
ordered starting from a lower register number at a lower stack
address (with the possible gap for aligning the stack at the top);
this is the opposite order that llvm normally produces.

To achieve this, reverse the order of the registers in the
assignCalleeSavedSpillSlots callback, to get the stack objects
laid out by PrologEpilogInserter in the right order, and adjust
computeCalleeSaveRegisterPairs to lay them out from the bottom up.

This allows generated prologs more often to match the format that
allows the unwind info to be written as packed info.

Differential Revision: https://reviews.llvm.org/D88677