Treat custom textures with "_arb" suffix as having arbitrary mipmaps

This is adapted from Bighead's code that was posted at
https://forums.dolphin-emu.org/Thread-dolphin-custom-texture-mipmaps?pid=460867#pid460867

In master, custom textures are never treated as having arbitrary mipmaps,
so we need either a change like this or a change that makes us apply the
arbitrary mipmap heuristic even when a custom texture is used.

Source/Core/VideoCommon/HiresTextures.h

@@ -35,6 +35,8 @@ public:
   ~HiresTexture();
 
   AbstractTextureFormat GetFormat() const;
+  bool HasArbitraryMipmaps() const;
+
   struct Level
   {
     Level();
@@ -59,4 +61,5 @@ private:
   static std::string GetTextureDirectory(const std::string& game_id);
 
   HiresTexture() {}
+  bool m_has_arbitrary_mipmaps;
 };