Support MSVC-style specifiers on systems with limited availability of types of various sizes

[eyalroz]

Currently, MSVC-style size-explicit specfiers (%I8, %I16, %I32, %I64) are all-or-nothing: Either all are supported, or none are. But for all of them to be supported, we need to be able to pass 64-bit integer values, i.e. need a 64-bit-sized integer type; and that's not always available. This prevents us from enabling such support by default (#213). Let's modify things so that we can live with smaller sizes - 32-bit should probably be enough (although, who knows, right?)