[RGen] Use GetStruct on Dlfcn to retrieve CoreMedia.CMTime and AVCaptureWhiteBalanceGains

[mandel-macaque]

While writing the support for static fields we notice that bgen does not use the GetSturct method, but instead does the following:

./AVFoundation/AVCaptureDevice.g.cs:2684:                               return *((global::CoreMedia.CMTime *) Dlfcn.dlsym (Libraries.AVFoundation.Handle, "AVCaptureExposureDurationCurrent"));
./AVFoundation/AVCaptureDevice.g.cs:2825:                               return *((AVCaptureWhiteBalanceGains *) Dlfcn.dlsym (Libraries.AVFoundation.Handle, "AVCaptureWhiteBalanceGainsCurrent"));

We should use the GetStruct call but we have copied the behavior of bgen to make sure that we do not break it when porting to the roslyn code generator. Once we have all bindings we should rivets these two structs.