Upgrade Argdata to version 0.6.

In 0.5, the C++ bindings were broken on non-CloudABI systems. This went
by unnoticed, as we didn't use the C++ bindings on those systems yet.
Now that I'm preparing the import of CloudABI's networking daemon,
Flower, into the Ports tree, this is becoming problematic.

Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D12065