Mark debug sections referenced by kept debug sections

Mark debug sections referenced by kept debug sections

If a debug section is referenced by a kept debug section, it should
also be kept.

Some targets, like mips, keep input files when there are some special
sections, like .gnu.attributes, even if input file is unused otherwise.
In this case, all debug sections are kept.  The new test will fail on
such targets.  We can either fix those targets or XFAIL the test.

bfd/

	PR ld/20882
	* elflink.c (elf_gc_mark_debug_section): New function.
	(_bfd_elf_gc_mark_extra_sections): Mark any debug sections
	referenced by kept debug sections.

ld/

	PR ld/20882
	* testsuite/ld-gc/gc.exp: Run pr20882.
	* testsuite/ld-gc/pr20882.d: New file.
	* testsuite/ld-gc/pr20882a.s: Likewise.
	* testsuite/ld-gc/pr20882b.s: Likewise.
	* testsuite/ld-gc/pr20882c.s: Likewise.