64-bit length fields on ARM don't work

ARM only allows aligned accesses.  GCC generated unaligned accesses for the copy buffer, but ARM forces alignment, causing the first two bytes to be trampled.  I changed the mechanism to create two temporaries and memcpy them in.  It now works on ARM (and x86) for large websocket chunks.

mongoose.c

@@ -2962,9 +2962,10 @@ size_t mg_websocket_write(struct mg_connection *conn, int opcode,
     } else {
       // 64-bit length field
       copy[1] = 127;
-      * (uint32_t *) (copy + 2) = (uint32_t)
-        htonl((uint32_t) ((uint64_t) data_len >> 32));
-      * (uint32_t *) (copy + 6) = (uint32_t) htonl(data_len & 0xffffffff);
+      const uint32_t hi = htonl((uint32_t) ((uint64_t) data_len >> 32));
+      const uint32_t lo = htonl(data_len & 0xffffffff);
+      memcpy(copy+2,&hi,sizeof(hi));
+      memcpy(copy+6,&lo,sizeof(lo));
       memcpy(copy + 10, data, data_len);
       copy_len = 10 + data_len;
     }