ppc64le: Fix calling convention of external functions with > 8 parameters (RHBZ#1225995).
    
    For external (ie. C) functions with more than 8 parameters, we must
    pass the first 8 parameters in registers and then all the remaining
    parameters on the stack.
    
    Unfortunately the original backend copied the stack offset from ppc64,
    where it works, but the offset was wrong for ppc64le.
    
    By experimentation I found the correct offset.