core: Introduce default CPU exception handler

The CPU session interfaces comes with the ability to install an
exception handler per thread. This patch enhances the feature with the
provision of a default signal handler that is used if no thread-specific
handler is installed. The default signal handler can be set by
specifying an invalid thread capability and a valid signal context
capability.

Furthermore, this patch relaxes the requirement of the order of the
calls of 'exception_handler' and 'set_pager'. Originally, the exception
handler could be installed not before setting a pager. Now, we remember
the installed exception handler in the 'Cpu_thread' and propagate to to
the platform thread at a later time.

base/include/rm_session/rm_session.h

@@ -166,6 +166,11 @@ namespace Genode {
 
 		/**
 		 * Register signal handler for region-manager faults
+		 *
+		 * On Linux, this signal is never delivered because page-fault handling
+		 * is performed by the Linux kernel. On microkernel platforms,
+		 * unresolvable page faults (traditionally called segmentation fault)
+		 * will result in the delivery of the signal.
 		 */
 		virtual void fault_handler(Signal_context_capability handler) = 0;