Typically flight hardware that goes up there is characterized completely. It is not unusual to use decade old GCC versions because their behaviour is completely

Typically flight hardware that goes up there is characterized completely. It is not unusual to use decade old GCC versions because their behaviour is completely understood in-house, and all the required functionality is coded from scratch rather than importing a library from github. The restrictions (power/computational power) are very high and reliability is the major concern for this kind of systems, making development patterns like that absolutely necessary. Arduino is fine for production if you are doing something (relatively) simple terrestrially, but space is a totally different beast.