libretro: add selectable system-clock RTC source

[duonqfs]

Summary

Adds an RTC Time Source core option:

• Deterministic: preserves the existing baseline-plus-frame-counter behavior.
• System Clock: reads the host wall clock, including time elapsed while
  content is suspended or not running.

Deterministic remains the default to preserve existing savestate, movie,
and netplay behavior.

Motivation

Always using deterministic emulated time is useful for reproducibility, but
time-based cartridge events do not advance while the emulator is stopped.
This is particularly noticeable on handheld devices used for Pokémon games.

This is related to libretro/gpsp#275. It does not change RTC cartridge autodetection;
the existing RTC Support option still controls whether RTC hardware is enabled.

Testing

• System-clock behavior tested through the NextUI on TRIMUI Brick Pro.