mach-o: fix section number while writing symbols.

bfd/
	* mach-o.c (bfd_mach_o_mangle_symbols): Use index from
	output_section.
	(bfd_mach_o_build_seg_command): Add comment.  Realign segment.
	Fix style.
	(bfd_mach_o_build_commands, bfd_mach_o_read_thread): Fix style.