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.

test/link/rom0-tiny-no-t.out

@@ -1,2 +1,2 @@
-warning: Section "rom" is bigger than the max size for that type: 0x8000 > 0x4000
-error: Sanity checks failed
+error: Section "rom" is bigger than the max size for that type: 0x8000 > 0x4000
+Linking failed with 1 error

test/link/vram-fixed-dmg-mode-d.out

@@ -1,2 +1,2 @@
-warning: v1: VRAM bank 1 can't be used with option -d
-error: Sanity checks failed
+error: v1: VRAM bank 1 can't be used with option -d
+Linking failed with 1 error