ld: Xfail PR ld/26936 test if not supported

ld: Xfail PR ld/26936 test if not supported

Linkonce sections and comdat groups can be mixed only if comdat groups
have only a single member with matching symbol table entries.  Xfail
ld/26936 test:

1. If comdat groups always have more than one member.
2. If symbol table entries in linkonce and comdat group don't match.
3. If the assembly source file is renamed.

	PR ld/26936
	* testsuite/ld-elf/pr26936.d: Xfail targets which don't support
	mixing linkonce and comdat sections.