Single threaded standard c library stuff should port between the two no problem.
OpenGL to Mesa should be fairly trivial.
For demo coding (since the point is to squeeze as much out of the machine as you can and to proove what a smarty pants you are) there is always room for a little assembly, but most everything can be written in C (and probably should be for a first attempt, then optimise the critical bits).
Once you are programming the GUI or you are writing programs with multiple threads, and message passing and etc. you are getting intimate with the operating system and then the APIs are quite different. Many of the concepts are shared, but the spelling is quite different.
