ld, testsuite: only run CTF tests when ld and GCC support CTF

ld, testsuite: only run CTF tests when ld and GCC support CTF

The CTF testsuite runs GCC to generate CTF that it knows matches the
input .c files before doing a run_dump_test over it.  So we need a GCC
capable of doing that, and we need to always avoid running those tests
if libctf was disabled because the linker will never be capable of it.

ld/
	* configure.ac (enable_libctf): Substitute it.
	* Makefile.am (enablings.exp): New.
	(EXTRA_DEJAGNU_SITE_CONFIG): Add it.
	(DISTCLEANFILES): Likewise.
	* Makefile.in: Regenerate.
	* configure: Likewise.
	* testsuite/lib/ld-lib.exp (compile_one_cc): New.
	(check_ctf_available): Likewise.
	(skip_ctf_tests): Likewise.
	* testsuite/ld-ctf/ctf.exp: Call skip_ctf_tests.