Expose SRAM pointers for frontends that manage save data their own way (#1643)

* Add a clarifying comment

- In case it saves some poor bastard hours of fruitless work

* Expose GBA and NDS save memory

- Add GetSaveMemory and GetSaveMemoryLength functions
- Where unsupported, they return null and zero

src/RTC.cpp

@@ -30,6 +30,9 @@ using Platform::LogLevel;
 namespace RTC
 {
 
+/// This value represents the Nintendo DS IO register,
+/// \em not the value of the system's clock.
+/// The actual system time is taken directly from the host.
 u16 IO;
 
 u8 Input;