Make RGBLINK able to link SDCC object files

This requires a LOT of tricky code, mostly due to the format itself being,
er, not the most straightforward.
Everything is converted to existing RGBLINK concepts (sections, patches,
etc.), so the core code is essentially unchanged.
(A couple of genuine RGBLINK bugs were uncovered along the way, so some of
the core code *is* changed, notably regarding `SECTION FRAGMENT`s.)

All of this code was clean-roomed, so SDCC's GPLv2 license does not apply.

include/link/script.h

@@ -11,11 +11,13 @@
 #define RGBDS_LINK_SCRIPT_H
 
 #include <stdint.h>
+#include "linkdefs.h"
 
 extern FILE * linkerScript;
 
 struct SectionPlacement {
 	struct Section *section;
+        enum SectionType type;
 	uint16_t org;
 	uint32_t bank;
 };