Kernel/Loader: Grab the system mode from the NCCH ExHeader.

3dsx and elf files default to system mode 2 (96MB allocated to the application).

This allows Home Menu to boot without modifications.
Closes #1849

src/core/system.h

@@ -15,7 +15,7 @@ enum class Result {
     ErrorInitVideoCore, ///< Something went wrong during video core init
 };
 
-Result Init(EmuWindow* emu_window);
+Result Init(EmuWindow* emu_window, u32 system_mode);
 bool IsPoweredOn();
 void Shutdown();
 }