extend.texi (Cilk Plus Builtins): cilkplus.org now uses https by default.

        * doc/extend.texi (Cilk Plus Builtins): cilkplus.org now uses
        https by default.
        * doc/passes.texi (Cilk Plus Transformation): Ditto.
        * doc/generic.texi (Statements for C++): Ditto, and use @uref.

From-SVN: r243962