How should _Decimal64 and _Decimal128 be aligned on stack?

[Ben Elliston]

> DFP is beyond i386 psABI. Gcc aligns _Decimal32 to 4 byte, _Decimal64 to 8 bytes
> and _Decimal128 to 16bytes. The question is what is the best alignment for them
> when passing to a functions.

The original work I did for the x86-64 backend placed them at that
alignment because that is the required alignment for loading those
values into SSE registers.  Right?

Ben