GC: Also check the local debug definition section

GC: Also check the local debug definition section

Extend

commit b7c871edcd83ccdc5fcd8148a7f433efd6b52255
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Wed May 17 07:57:15 2017 -0700

    Mark debug sections referenced by kept debug sections

to handle the local debug definition section.

bfd/

	PR ld/20882
	* elflink.c (elf_gc_mark_debug_section): Also check the local
	debug definition section.

ld/

	PR ld/20882
	* testsuite/ld-gc/pr20882.d: Also dump and check .debug_abbrev
	section.
	* testsuite/ld-gc/pr20882b.s: Add .debug_abbrev section.