Make linker-created symbols relocatable where appropriate.

Make linker-created symbols relocatable where appropriate.

Linker-created symbols like __ehdr_start, __etext, __edata, and end
should be relocatable, not absolute.

gold/
	* output.cc (Output_segment::first_section): Return NULL if there are
	no sections in the segment.
	* output.h (Output_segment::first_section_load_address): Assert that
	first section is not NULL.
	* symtab.cc (Symbol_table::sized_write_globals): Attach linker-created
	segment-relative symbols to first section of the segment.