Run certain ld-plugun lto tests only for ELF

lto-6.c refers to _etext, pr12760b.c has a .gnu.warning section,
the pr23818 test uses readelf, and pr23958.c has a .symver directive.

	* testsuite/ld-plugin/lto.exp (lto_link_tests): Move lto-6,
	pr12760, pr23818 and pr23958 tests to..
	(lto_link_elf_tests): ..here.