hw: adjust core's signal thread's stack size

The stack size should be architecture dependent.

Fix #1854

repos/base-hw/src/core/include/signal_root.h

@@ -30,7 +30,7 @@ namespace Genode
 	 */
 	class Signal_handler
 	{
-		enum { STACK_SIZE = 4096 };
+		enum { STACK_SIZE = 1024 * sizeof(addr_t) };
 
 		Rpc_entrypoint _entrypoint;