Don't link gdb twice against libiberty
    
    I noticed that gdb includes libiberty twice in its link line.  I don't
    think there's a need for this, so this patch removes one of the
    references.
    
    gdb/ChangeLog
    2020-01-14  Tom Tromey  <tom@tromey.com>
    
    	* Makefile.in (CLIBS): Remove second use of $(LIBIBERTY).
    
    Change-Id: I43bb7100660867081f937c67ea70ff751c62bbfb