x/mobile: log skipped functions and variables to console

[Federico Bond]

x/mobile adds comments to the generated bridge source when a function or variable has an unsupported type and thus cannot be correctly bridged. No indication is given in the console output of gomobile that this has happened though, forcing devs to look at the actual generated code to find out why some function or field/variable is missing.

It would be nice for gomobile to log these when calling the gomobile bind command. A simple log message should suffice in most cases, but a future extension could add a strict mode option and fail the compilation if any of these problems is found.