AArch64: __va_list.__stack must be 8-byte aligned

The va_start macro for AArch64 must set va_list.__stack to the address
following the last named argument on the stack, rounded up to an alignment
of 8 bytes.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201797 91177308-0d34-0410-b5e6-96231b3b80d8