Deprecate DEF-less definitions (#1193)

test/asm/symbol-override.asm

@@ -1,14 +1,14 @@
-V = 0
-V = 1
+def V = 0
+def V = 1
     PRINTLN "V={V}"
 
-W equ 1
-W = 0
+def W equ 1
+def W = 0
 
 rsset 1
-X rb 0
-X = 0
+def X rb 0
+def X = 0
 
 SECTION "Test", ROM0[1]
 Y:
-Y = 0
+def Y = 0