Add info about string expansions in error reports

This is especially useful when an EQUS expands to another one, to help
track them.
This is done separately from the file stack as the EQUS stack is separate
(which is itself because EQUS are managed *way* differently).

src/asm/fstack.c

@@ -262,6 +262,20 @@ void fstk_Dump(void)
 	fprintf(stderr, "%s(%d)", tzCurrentFileName, nLineNo);
 }
 
+/*
+ * Dump the string expansion stack to stderr
+ */
+void fstk_DumpStringExpansions(void)
+{
+	const struct sStringExpansionPos *pExpansion = pCurrentStringExpansion;
+
+	while (pExpansion) {
+		fprintf(stderr, "while expanding symbol \"%s\"\n",
+			pExpansion->tzName);
+		pExpansion = pExpansion->pParent;
+	}
+}
+
 /*
  * Extra includepath stuff
  */