libsys: add Tomas Hruby's asynsend barrier

This barrier ensures that all fields of an asynchronously sent
message are properly initialized before the message is marked as
valid.

Change-Id: I7b9590c11c4e040c8f992f1dd2581e54201bf214

minix/lib/libsys/asynsend.c

@@ -126,6 +126,7 @@ int fl;
   fl |= AMF_VALID;	/* Mark in use */
   msgtable[next_slot].dst = dst;
   msgtable[next_slot].msg = *mp;
+  __insn_barrier();
   msgtable[next_slot].flags = fl;		/* Has to be last. The kernel 
 					 	 * scans this table while we
 						 * are sleeping.