[llvm] [AArch64] Roundup newly allocated stack size according -mstack-alignment (PR #100859)

[via llvm-commits]

vfdff wrote:

> Does this option mean that the stack must be aligned to N into and out of any function being compiled? That sounds like it creates a new ABI that would not necessarily be followed by any external routines.

Yes, I expected the stack can be aligned  to N  with a option **-mstack-alignment=N** or similar, and it keep same as now if the N is 16, which is similar to that of x86 target, https://gcc.godbolt.org/z/xW6z8b85d.
It is expected to fix some unaligned spill for sve load/store if user add the option **-mstack-alignment=N** later.

https://github.com/llvm/llvm-project/pull/100859