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/loader/ncch.h

@@ -185,6 +185,13 @@ public:
      */
     ResultStatus Load() override;
 
+    /**
+     * Loads the Exheader and returns the system mode for this application.
+     * @param system_mode Out variable where the system mode will be stored.
+     * @return ResultStatus result of the operation
+     */
+    ResultStatus LoadKernelSystemMode(u32& system_mode);
+
     /**
      * Get the code (typically .code section) of the application
      * @param buffer Reference to buffer to store data