Validate new toArgTypes()-based System V implementation against
previous version.
fix
Get rid of obsolete DtoVariadicArgument().
Refactoring: move ABI-specific variadic stuff to TargetABI type.
System V AMD64 ABI: don't use CompositeToInt rewrite.

Hope for toArgTypes() to get it right and otherwise be compatible with DMD.
SysV AMD64 ABI: don't rewrite to a single bit.

Otherwise std.uni cannot be compiled:
getAbiType(BitPacked!(bool, 1LU)) mismatch: i1 vs. i8
gen/tollvm.cpp:527: void DtoStore(llvm::Value*, llvm::Value*): Assertion `src->getType() != llvm::Type::getInt1Ty(gIR->context()) && "Should store bools as i8 instead of i1."' failed.