Don't write zeros to a gap in the output file

Writing out zeros is counterproductive if a file system supports
sparse files.  A very large gap need not take much actual disk space,
but it usually will if zeros are written.

memory_bseek also supports not writing out zeros in a gap.

	* elf.c (write_zeros): Delete.
	(assign_file_positions_for_load_sections): Don't call write_zeros.
	Comment.