Evaluate __start_* and __stop_* symbol PROVIDE expressions early

Evaluate __start_* and __stop_* symbol PROVIDE expressions early

Makes these symbols defined before bfd_elf_size_dynamic_sections, to
avoid horrible hacks elsewhere.  The exp_fold_tree undefweak change
is necessary to define undefweak symbols early too.  The comment was
wrong.  PROVIDE in fact defines undefweak symbols, via
bfd_elf_record_link_assignment.

	PR ld/19175
	* ldlang.c (lang_insert_orphan): Evaluate __start_* and __stop_*
	symbol PROVIDE expressions.
	* ldexp.c (exp_fold_tree_1 <etree_provide>): Define undefweak
	references.