Fix bug #49827: wrong cast to size_t on 16-bit x86 architecture

I hope I caught all of them.
TODO: Same for casts to get rid of alignment warnings, these are also casts via size_t

src/api/sockets.c

@@ -1279,7 +1279,7 @@ lwip_writev(int s, const struct iovec *iov, int iovcnt)
   msg.msg_namelen = 0;
   /* Hack: we have to cast via number to cast from 'const' pointer to non-const.
      Blame the opengroup standard for this inconsistency. */
-  msg.msg_iov = (struct iovec *)(size_t)iov;
+  msg.msg_iov = LWIP_CONST_CAST(struct iovec *, iov);
   msg.msg_iovlen = iovcnt;
   msg.msg_control = NULL;
   msg.msg_controllen = 0;

src/apps/lwiperf/lwiperf.c

@@ -294,7 +294,7 @@ lwiperf_tcp_client_send_more(lwiperf_state_tcp_t* conn)
     } else {
       /* transmit data */
       /* @todo: every x bytes, transmit the settings again */
-      txptr = (void*)(size_t)&lwiperf_txbuf_const[conn->bytes_transferred % 10];
+      txptr = LWIP_CONST_CAST(void*, &lwiperf_txbuf_const[conn->bytes_transferred % 10]);
       txlen_max = TCP_MSS;
       if (conn->bytes_transferred == 48) { /* @todo: fix this for intermediate settings, too */
         txlen_max = TCP_MSS - 24;

src/core/def.c

@@ -103,11 +103,11 @@ lwip_strnstr(const char* buffer, const char* token, size_t n)
   const char* p;
   int tokenlen = (int)strlen(token);
   if (tokenlen == 0) {
-    return (char *)(size_t)buffer;
+    return LWIP_CONST_CAST(char *, buffer);
   }
   for (p = buffer; *p && (p + tokenlen <= buffer + n); p++) {
     if ((*p == *token) && (strncmp(p, token, tokenlen) == 0)) {
-      return (char *)(size_t)p;
+      return LWIP_CONST_CAST(char *, p);
     }
   }
   return NULL;

src/core/pbuf.c

@@ -1119,7 +1119,7 @@ pbuf_skip_const(const struct pbuf* in, u16_t in_offset, u16_t* out_offset)
 struct pbuf*
 pbuf_skip(struct pbuf* in, u16_t in_offset, u16_t* out_offset)
 {
-  return (struct pbuf*)(size_t)pbuf_skip_const(in, in_offset, out_offset);
+  return LWIP_CONST_CAST(struct pbuf*, pbuf_skip_const(in, in_offset, out_offset));
 }
 
 /**

src/core/timeouts.c

@@ -179,7 +179,7 @@ void sys_timeouts_init(void)
   for (i = 1; i < LWIP_ARRAYSIZE(lwip_cyclic_timers); i++) {
     /* we have to cast via size_t to get rid of const warning
       (this is OK as cyclic_timer() casts back to const* */
-    sys_timeout(lwip_cyclic_timers[i].interval_ms, cyclic_timer, (void*)(size_t)&lwip_cyclic_timers[i]);
+    sys_timeout(lwip_cyclic_timers[i].interval_ms, cyclic_timer, LWIP_CONST_CAST(void*, &lwip_cyclic_timers[i]));
   }
 
   /* Initialise timestamp for sys_check_timeouts */

src/include/lwip/arch.h

@@ -133,6 +133,11 @@ typedef uintptr_t mem_ptr_t;
 #endif
 #endif
 
+/** C++ const_cast<target_type>(val) equivalent to remove constness from a value */
+#ifndef LWIP_CONST_CAST
+#define LWIP_CONST_CAST(target_type, val) ((target_type)((ptrdiff_t)val))
+#endif
+
 /** Allocates a memory buffer of specified size that is of sufficient size to align
  * its start address using LWIP_MEM_ALIGN.
  * You can declare your own version here e.g. to enforce alignment without adding