Update a symbol's filename and line when defined

Currently, all symbols are assigned a filename and line when they're
first encountered and added to the internal hash table. This is often
not expected and leads to erroneous error messages.

test/asm/label-redefinition.asm

@@ -0,0 +1,7 @@
+SECTION "sec", ROM0
+    dw Sym
+m: MACRO
+Sym::
+ENDM
+    m
+Sym::

test/asm/label-redefinition.out

@@ -0,0 +1,3 @@
+ERROR: label-redefinition.asm(7):
+    'Sym' already defined in m(6)
+error: Assembly aborted in pass 1 (1 errors)!