-y support

* ld.texinfo: new doc.
	* ldgram.y, ldlex.l: understand -y<symbol>
	* ldmain.c (Q_enter_file_symbols): if had -y, lookup symbol and
	print info. (add_ysym): new function.
	* ldsym.h: (ldsym_type): new define SYM_Y.

ld/ldsym.h

@@ -56,6 +56,8 @@ typedef struct user_symbol_struct
   /* If this symbol explicitly should be kept, despite discarding
      most others.  */
 #define SYM_KEEP 8
+  /* If its got -y set */
+#define SYM_Y 16
   int flags;
 } ldsym_type;