IO: made stream classes directly use base-class ctor instead of init().¹

1: std::ios_base::init() simply assigns some values, so this is safe.
Also, the default ctor for the basic_stream classes are implicitly
deleted due to the (basic_streambuf*) ctor, so there's that.
test/premake: added stdlib option for Linux.¹

1: Defaults to libstdc++.