[AArch64][SVE] Don't align the last SVE callee save.

Instead of aligning the last callee-saved-register slot to the stack
alignment (16 bytes), just align the SVE callee-saved block. This also
simplifies the code that allocates space for the callee-saves.

This change is needed to make sure the offset to which the callee-saved
register is spilled, corresponds to the offset used for e.g. unwind call
frame instructions.

Reviewers: efriedma, paulwalker-arm, david-arm, rengolin

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D84042