Include undefined symbols in the symbol table for listing purposes.

MACRO11 V05.05 does this:

```
.MAIN.  MACRO V05.05  Sunday 18-APR-2021 16:29  Page 1

      1
      2 000000  012700  000000G                 mov #lab1,r0
      3
      4                                         .dsabl gbl
      5
U     6 000004  012700  000000                  mov #lab2,r0
      7
      8         000001                          .end

.MAIN.  MACRO V05.05  Sunday 18-APR-2021 16:29  Page 1-1
Symbol table

LAB1  = ****** GX       LAB2  = ******

. ABS.  000000    000   (RW,I,GBL,ABS,OVR)
        000010    001   (RW,I,LCL,REL,CON)
Errors detected:  1

*** Assembler statistics

Work  file  reads: 0
Work  file writes: 0
Size of work file: 34 Words  ( 1 Pages)
Size of core pool: 9260 Words  ( 35 Pages)
Operating  system: RSX-11M/M-PLUS

Elapsed time: 00:00:00.01
GBL,GBL/-SP=GBL
```

tests/RunTests

@@ -16,6 +16,7 @@ TESTS="test-asciz \
     test-enabl-lcm \
     test-endm \
     test-float \
+    test-gbl \
     test-if \
     test-impword \
     test-include \

tests/test-gbl.lst.ok

@@ -0,0 +1,26 @@
+       1                                ;;;;
+       2                                ;
+       3                                ; Test that .dsabl gbl at the end of the source does not carry
+       4                                ; over to the beginning of the next pass.
+       5                                ; This is actually guaranteed automatically, since it is used in pass 1 only.
+       6                                ; The symbol should however be entered into the symbol table as UNKNOWN.
+       7                                ;
+       8                                
+       9 000000 012700  000000G                 mov     #lab1,r0        ; ok: implicitly global (imported)
+      10                                        .dsabl  gbl
+test-gbl.mac:11: ***ERROR Invalid expression (complex relocation)
+      11 000004 012700  000000                  mov     #lab2,r0        ; error: undefined
+      12                                
+      13                                        .end
+      13                                
+
+
+Symbol table
+
+.       ******R      001 LAB1  = ****** GX        LAB2  = ******           
+
+
+Program sections:
+
+. ABS.  000000    000   (RW,I,GBL,ABS,OVR,NOSAV)
+        000010    001   (RW,I,LCL,REL,CON,NOSAV)

tests/test-gbl.mac

@@ -0,0 +1,13 @@
+;;;;
+;
+; Test that .dsabl gbl at the end of the source does not carry
+; over to the beginning of the next pass.
+; This is actually guaranteed automatically, since it is used in pass 1 only.
+; The symbol should however be entered into the symbol table as UNKNOWN.
+;
+
+        mov     #lab1,r0        ; ok: implicitly global (imported)
+        .dsabl  gbl
+        mov     #lab2,r0        ; error: undefined
+
+        .end