settings: Add custom RTC settings

Stored as signed seconds since epoch.

src/core/settings.h

@@ -350,6 +350,9 @@ struct Values {
     bool use_docked_mode;
     bool enable_nfc;
     std::optional<u32> rng_seed;
+    std::optional<s64> custom_rtc; // Measured in seconds since epoch
+    s64 custom_rtc_differential;   // Set on game boot, reset on stop. Seconds difference between
+                                   // current time and `custom_rtc`
     s32 current_user;
     s32 language_index;