vvp: Reduce overhead of passing vvp_net_ptr_t between functions

vvp_net_ptr_t uses vvp_sub_pointer_t to implement a tagged pointer with the
tag containing the port number.

The size of the tagged pointer is that of a normal pointer and could easily
be passed in a register when passing it as an argument to a function.

But since the vvp_sub_pointer_t type has a non-standard destructor it is
instead passed on the stack with the register containing a pointer to the
stack location where the value is stored.

This creates extra boiler plate code when passing a vvp_net_ptr_t to a
function writing and reading the value to and from the stack.

Use the default destructor for vvp_sub_pointer_t to avoid this and have the
value passed in a register.

There isn't much of a performance gain but the change is simple enough to
do anyway.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>

vvp/vvp_net.h

@@ -1055,7 +1055,7 @@ template <class T> class vvp_sub_pointer_t {
 	    bits_ |= port__;
       }
 
-      ~vvp_sub_pointer_t() { }
+      ~vvp_sub_pointer_t() = default;
 
       T* ptr()
       { return reinterpret_cast<T*> (bits_ & ~UINTPTR_C(3)); }