PR22836, "-r -s" doesn't work with -g3 using GCC 7

PR22836, "-r -s" doesn't work with -g3 using GCC 7

This fixes the case where all of a group is removed with ld -r, the
situation in the PR, and failures where part of a group is removed
that contain relocs.

bfd/
	PR 22836
	* elf.c (_bfd_elf_fixup_group_sections): Account for removed
	relocation sections.  If size reduces to just the flag word,
	remove that too and mark with SEC_EXCLUDE.
	* elflink.c (bfd_elf_final_link): Strip empty group sections.
binutils/
	* testsuite/binutils-all/group-7.s,
	* testsuite/binutils-all/group-7a.d,
	* testsuite/binutils-all/group-7b.d,
	* testsuite/binutils-all/group-7c.d: New tests.
	* testsuite/binutils-all/objcopy.exp: Run them.
ld/
	* testsuite/ld-elf/pr22836-2.d,
	* testsuite/ld-elf/pr22836-2.s: New test.