.sym files miss some symbols from multiple SECTION UNIONs/FRAGMENTs

[Rangi42]

46d6652 fixed this issue for .map files but not .sym files.

The test/link/load-fragment test case demonstrates this.

$ rgbasm -E -o a.o a.asm
$ rgbasm -E -o b.o b.asm
$ rgblink -x -o ab.gb -m ab.map -n ab.sym a.o b.o

ab.map includes Label:

SRAM bank #0:
  SECTION: $a000-$a004 ($0005 bytes) ["test"]
           $a003 = Label
    SLACK: $1ffb bytes

but ab.sym is empty.