Save object file name of each symbol in linker

This is useful to generate error messages when there is a symbol that
appears in more than one object file.

Signed-off-by: Antonio Niño Díaz <antonio_nd@outlook.com>

include/link/mylink.h

@@ -89,6 +89,7 @@ struct sSymbol {
 	SLONG nSectionID;	/* internal to object.c */
 	struct sSection *pSection;
 	SLONG nOffset;
+	char *pzObjFileName; /* Object file where the symbol is located. */
 	char *pzFileName; /* Source file where the symbol was defined. */
 	ULONG nFileLine; /* Line where the symbol was defined. */
 };