target_ops/C++: GNU/Linux + x86/AMD64
    
    This converts the x86/AMD64 GNU/Linux ports to C++ified target_ops.
    
    - inf-child/"native" is converted to an actual base class
      (inf_child_target), that is inherited by the linux-nat target.
      inf_child_target will be inherited by all other native targets too,
      of course, in the following patches.
    
    - The old weird double-target linux_ops mechanism in linux-nat.c, is
      gone, replaced by adding a few virtual methods to linux-nat.h's
      target_ops, called low_XXX, that the concrete linux-nat
      implementations override.  Sort of like gdbserver's
      linux_target_ops, but simpler, for requiring only one
      target_ops-like hierarchy, which spares implementing the same method
      twice when we need to forward the method to a low implementation.
      The low target simply reimplements the target_ops method directly in
      that case.
    
      There are a few remaining linux-nat.c hooks that would be better
      converted to low_ methods like above too.  E.g.:
    
       linux_nat_set_new_thread (t, x86_linux_new_thread);
       linux_nat_set_new_fork (t, x86_linux_new_fork);
       linux_nat_set_forget_process
    
      That'll be done in a follow up patch.
    
    - We can no longer use functions like x86_use_watchpoints to install
      custom methods on an arbitrary base target.  The patch replaces that
      with a template mixin.  See x86_nat_target in x86-nat.h.  That will
      be used by other non-Linux x86 targets in following patches.
    
    - linux-thread-db is converted here too.