Simplify the VAArg code slightly.  Fix (I hope!) i32 va_arg argument lowering in the sandbox ABI.

Note: In the non-sandbox ABI, va_args lowering is wrong for either the first 4 or subsequent i32 arguments (upstream as well).  The ABI document is woefully inadequate in describing how variadic functions work, so I’ll need to see what GCC does to see which is correct.