tree.c (build_common_builtin_nodes): Do not use TM_PURE attribute unless language has support for TM.

        PR/51291
        * tree.c (build_common_builtin_nodes): Do not use TM_PURE
        attribute unless language has support for TM.
        * config/i386/i386.c (ix86_init_tm_builtins): Exit gracefully in
        the absence of TM builtins.

From-SVN: r182179