Detect GC region based on the actual region value

The county code isn't 100% reliable for detecting the region.
For instance, some games released in Korea have the country
code E even though they're region-locked to NTSC-J consoles.

This commit makes the GC disc region detection match the Wii
disc region detection (apart from the region value being in
a different place on the disc).

Source/Core/DiscIO/Enums.h

@@ -40,10 +40,10 @@ enum class Country
   NUMBER_OF_COUNTRIES
 };
 
-// Regions 0 - 2 and 4 match Nintendo's Wii region numbering.
+// Regions 0 - 2 and 4 match Nintendo's GameCube/Wii region numbering.
 enum class Region
 {
-  NTSC_J = 0,          // Japan and Taiwan
+  NTSC_J = 0,          // Japan and Taiwan (and South Korea for GameCube only)
   NTSC_U = 1,          // Mainly North America
   PAL = 2,             // Mainly Europe and Oceania
   UNKNOWN_REGION = 3,  // 3 seems to be unused? Anyway, we need an UNKNOWN_REGION. Let's put it here