Prevent purging referenced symbols

This is an immediate fix for #492, although #342 is needed to implement the
desired functionality.

test/asm/purge-ref.asm

@@ -0,0 +1,23 @@
+SECTION "test", ROM0 ; Important: do not purge this!
+
+	dw ref
+	PURGE ref
+
+OK:
+	PURGE OK
+
+	dw NotOK
+NotOK:
+	PURGE NotOK
+
+EvenLessOK:
+	dw EvenLessOK
+	PURGE EvenLessOK
+
+SECTION "fixed", ROM0[0]
+
+Maybe:
+	dw Maybe
+	; This is currently fine because the expression above is fully evaluated
+	; above, so it's fine as of now... but changing it might make sense
+	PURGE Maybe

test/asm/purge-ref.err

@@ -0,0 +1,7 @@
+ERROR: purge-ref.asm(4):
+    Symbol "ref" is referenced and thus cannot be purged
+ERROR: purge-ref.asm(11):
+    Symbol "NotOK" is referenced and thus cannot be purged
+ERROR: purge-ref.asm(15):
+    Symbol "EvenLessOK" is referenced and thus cannot be purged
+error: Assembly aborted (3 errors)!