[Platform] [PC-98] Graphics GDC initialization

I've copy-pasted this snippet so many times, it's time it gets a proper
home.

Part of P0232, funded by [Anonymous].

platform/x86real/pc98/graph.cpp

@@ -0,0 +1,18 @@
+#include "platform.h"
+#include "x86real.h"
+#include "platform/x86real/pc98/graph.hpp"
+
+void graph_show_16color_400line(void)
+{
+	// Enter 400-line mode
+	_AH = 0x42;
+	_CH = 0xC0;
+	geninterrupt(0x18);
+
+	// Enter 16-color mode
+	outportb(0x6A, 1);
+
+	// Show VRAM
+	_AH = 0x40;
+	geninterrupt(0x18);
+}

platform/x86real/pc98/graph.hpp

@@ -0,0 +1,5 @@
+// Generic graphics layer functions
+// --------------------------------
+
+// Puts the graphics GDC into the common 16-color 640×400 graphics mode.
+void graph_show_16color_400line(void);