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

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