Use p_vaddr_offset to set p_vaddr on segments without sections

Use p_vaddr_offset to set p_vaddr on segments without sections

p_vaddr is currently set from the first section vma if a segment has
sections, and to zero if a segment has no sections.  This means we
lose p_vaddr when objcopy'ing executables if a segment without
sections has a non-zero p_vaddr.

This patch saves p_vaddr to p_vaddr_offset, and to make the use of
p_vaddr_offset consistent, inverts the sign.  (It's now added to
section vma to get segment vaddr, and added to zero when there are no
sections.)

	* elf.c (assign_file_positions_for_load_sections): Set p_vaddr
	from m->p_vaddr_offset for segments without sections.  Invert
	sign of p_vaddr_offset.
	(rewrite_elf_program_header, copy_elf_program_header): Save
	old segment p_vaddr to p_vaddr_offset.  Invert sign of
	p_vaddr_offset.