AArch64: __va_list.__stack must be 8-byte aligned

[Tim Northover]

Hi Oliver,

> The attached patch fixes an issue where the __stack pointer in a va_list
> struct was not rounded up to a multiple of 8 bytes by va_start.

It's a bit far-fetched, but I think this solution could fail
occasionally. The problem is that increasing the size of a byval
object will increase the size of the accesses involved in moving it
around. If the source happens to be the last thing on the last page
given to that process...

Did you look at modifying the call to CreateFixedObject in
SaveVarArgRegisters instead?

Cheers.

Tim.