ld -r: Don't merge with member of output section group

When doing a relocatable link, members of input section group are
placed in their own output sections.  We need to make sure that no
input sections are merged with member of output section group.

	PR ld/24819
	* emultempl/elf32.em (elf_orphan_compatible): Return FALSE for
	member of output section group when doing a relocatable link.
	* testsuite/ld-elf/pr24819.d: New file.
	* testsuite/ld-elf/pr24819.s: Likewise.