elf: Get the real kept section

When mixing linkonce and comdat sections, we need to keep searching to
get the real kept section.

bfd/

	PR ld/26936
	* elflink.c (_bfd_elf_check_kept_section): Get the real kept
	section.

ld/

	PR ld/26936
	* testsuite/ld-elf/pr26936.d: New file.
	* testsuite/ld-elf/pr26936a.s: Likewise.
	* testsuite/ld-elf/pr26936b.s: Likewise.
	* testsuite/ld-elf/pr26936c.s: Likewise.