Don't swap real and imaginary parts of complex numbers.

Rely on LLVM to select a sane order, don't use DMD as reference anymore.
Get rid of CfloatToInt ABI rewrite and extend CompositeToInt.

CompositeToInt now also accepts complex number pairs and all other types,
not just pointers to structs/static arrays.
Use single ByVal ABI rewrite for both Win64 and PPC.
Implement rewriteArgument() for PPC ABI.
Refactor Win64 ABI.

I've decided to use a consistent scheme here - types are returned the same
way as they are passed as arguments. So cfloat as 64-bit struct in RAX,
cdouble/creal and 80-bit real/ireal as types > 64 bit via sret.

MS allows using the x87 stack, but there are no official guidelines as
they don't use x87 at all.
Implement rewriteArgument() for Intrinsic ABI.
Daring, untested attempt to simplify the System V AMD64 ABI.

Rely on LLVM to assign the registers, for D linkage too.