Reuse __unwind_single_frame where appropriate

Reuse __unwind_single_frame where appropriate

        * exceptions.cc (__unwind_single_frame): Move up in file to be
        accessible from other places.  Move comment to getcontext.
        (stack_info::walk): Call __unwind_single_frame in 64 bit case.  Fix
        preceeding comment.
        (myfault_altstack_handler): Call __unwind_single_frame.
        (getcontext): Give comment from __unwind_single_frame a new home.
        (swapcontext): Fix comment.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>