Refactor RTC subsystem [WIP]

[meeq]

1. Optimizes away all RTC read operations except the initial sync and recalculates the date/time based on sync point using ticks.
2. Support for "Software RTC with no persistence": if your flashcart doesn't have an RTC, just set the time and we'll count forward from there.
3. Support for switching between clock sources: Joybus and 64DD (highly experimental)
4. Deprecates rtc_time_t struct in favor of standard C time types.
5. Adds support for settimeofday so standard C APIs can be used to get and set RTC

Still left to do:

• [ ] Update documentation
• [x] Strip out the 64DD RTC stuff (it's not fully-baked yet)
• [ ] Document SummerCart quirks
• [x] Handling the 2038 problem?