Fix handling of __ehdr_start when it cannot be defined.

Fix handling of __ehdr_start when it cannot be defined.

gold/
	* defstd.cc (in_segment): Define __ehdr_start here...
	* layout.cc (Layout::finalize): ...Instead of here.  Set the
	output segment when known.
	* resolve.cc (Symbol::override_base_with_special): Remember
	the original binding.
	* symtab.cc (Symbol::set_output_segment): New function.
	(Symbol::set_undefined): New function.
	* symtab.h (Symbol::is_weak_undefined): Check original undef
	binding.
	(Symbol::is_strong_undefined): New function.
	(Symbol::set_output_segment): New function.
	(Symbol::set_undefined): New function.
	* target-reloc.h (is_strong_undefined): Remove.
	(issue_undefined_symbol_error): Call Symbol::is_weak_undefined.
	Check for hidden undefs.
	(relocate_section): Call Symbol::is_strong_undefined.

	* testsuite/Makefile.am (ehdr_start_test_1)
	(ehdr_start_test_2, ehdr_start_test_3)
	(ehdr_start_test_4, ehdr_start_test_5): New test cases.
	* testsuite/Makefile.in: Regenerate.
	* testsuite/ehdr_start_def.cc: New source file.
	* testsuite/ehdr_start_test.cc: New source file.
	* testsuite/ehdr_start_test.t: New linker script.
	* testsuite/ehdr_start_test_4.sh: New shell script.