[LibOS] Align char msg[] in shim_ipc_msg to 16 bytes

The msg field of 'struct shim_ipc_msg' will for example be cast to
'struct shim_ipc_sysv_tellkey*' (in ipc_sysv_tellkey_callback()) and
needs to be properly aligned. There are also casts to other IPC-related
structures that also require alignment.

LibOS/shim/include/shim_ipc.h

@@ -101,7 +101,9 @@ struct shim_ipc_msg {
     size_t size;
     IDTYPE src, dst;
     unsigned long seq;
-    char msg[];
+    /* msg is used to store and read various structures, we need to ensure its proper alignment */
+    // TODO: this is only a temporary workaround until we rewrite the IPC subsystem.
+    char msg[] __attribute__((aligned(16)));
 } __attribute__((packed));
 
 struct shim_ipc_port;