frame.h: Update some comments.

frame.h: Update some comments.

        * frame.h: Update some comments.
        * defaults.h (TARGET_ATTRIBUTE_WEAK): Define.
        * crtstuff.c (__register_frame_info, __deregister_frame_info): Declare
        using TARGET_WEAK_ATTRIBUTE.
        (__do_global_dtors_aux): Check if __deregister_frame_info is
        zero before calling it.
        (__do_global_dtors): Likewise.
        (frame_dummy): Check if __register_frame_info is zero before
        calling it.
        (__frame_dummy): Likewise.

Co-Authored-By: Jeffrey A Law <law@cygnus.com>
From-SVN: r25487