* scripttempl/elf.sc: Use PROVIDE to define etext, edata, and end.

	* scripttempl/elf.sc: Use PROVIDE to define etext, edata, and end.

	Add a new script operator, PROVIDE, to define a symbol only if it
	is needed.
	* ldgram.y (PROVIDE): New token.
	(assignment): Accept PROVIDE.
	* ldlex.l (PROVIDE): New token.
	* ldexp.h (node_type): Add etree_provide to node_class enum.
	(exp_provide): Declare.
	* ldexp.c (exp_fold_tree): Handle etree_provide.
	(exp_provide): New function.
	(exp_print_tree): Handle etree_provide.
	* ld.texinfo: Document PROVIDE.