Provide virtual target "cfi" for targets supporting CFI.

	Provide virtual target "cfi" for targets supporting CFI.
	* ld-elf/eh-frame-hdr.d: Replace target and xfail statements by single
	`target: cfi'.
	* ld-elf/eh-group.exp: Call check_as_cfi instead of is_elf_format.
	* ld-elf/eh-group1.s, elf/eh-group2.s: Use more compatible section
	flags prefix '%'.
	* ld-elf/eh5.d: Replace target statement by `target: cfi' with an Alpha
	exception.  Relax the `Code alignment factor' matching.
	* lib/ld-lib.exp: Rename istarget as istarget_ld.
	(istarget, check_as_cfi): New procedure.
	(run_dump_test): New comment for the virtual target `cfi'.