Assign section locations to all UNIONs/FRAGMENTs

Fixes the test case from #800

The `out.gb` output was corrected, since the two "test"
fragments have a different order in ROM than in SRAM.
It is effectively:

; ROM0[$0000], fragments ordered by size
    jr Label
    dw Label
    db 0

; SRAM[$a000], fragments ordered by .o order
    ds 1  ; db 0
    ds 2  ; jr Label
Label:    ; $a003
    ds 2  ; dw Label

src/link/assign.c

@@ -112,6 +112,13 @@ static inline void assignSection(struct Section *section,
 	section->org = location->address;
 	section->bank = location->bank;
 
+	// Propagate the assigned location to all UNIONs/FRAGMENTs
+	// so `jr` patches in them will have the correct offset
+	for (struct Section *next = section->nextu; next != NULL; next = next->nextu) {
+		next->org = section->org;
+		next->bank = section->bank;
+	}
+
 	nbSectionsToAssign--;
 
 	out_AddSection(section);

test/link/load-fragment/b.asm

@@ -1,7 +1,6 @@
-
 SECTION "SECTION2", ROM0
 LOAD FRAGMENT "test", SRAM
-    jr Label
+	jr Label
 Label:
 	dw Label
 ENDL