garbage collect debug sections when no alloc sections are kept

garbage collect debug sections when no alloc sections are kept

The pr20882 testcase fails on a number of targets that add attribute
or note sections to object files, and the default linker script says
those sections should be kept.  This patch changes --gc-sections to
drop debug and special sections like .comment when no SEC_ALLOC
section in an object file is kept.  The assumption is that debug
sections are describing code and data that will become part of the
final image in memory.

	* elflink.c (_bfd_elf_gc_mark_extra_sections): Don't keep
	debug and special sections when no non-note alloc sections in an
	object are kept.