Don't attempt to compress bss sections

It doesn't make sense to try to compress a section without contents
since those sections take no space on disk.  Compression can only
increase the disk image size.

	* coffgen.c (make_a_section_from_file): Exclude !SEC_HAS_CONTENTS
	sections from compression and decompression.
	* elf.c (_bfd_elf_make_section_from_shdr): Likewise.