Extend Cpu_session with thread-affinity API

This patch introduces the functions 'affinity' and 'num_cpus' to the CPU
session interface. The interface extension will allow the assignment of
individual threads to CPUs. At this point, it is just a stub with no
actual platform support.

base-linux/src/core/include/platform_thread.h

@@ -59,6 +59,7 @@ namespace Genode {
 			int           start(void *ip, void *sp) { return 0; }
 			int           state(Thread_state *state_dst) { return 0; }
 			const char   *name() { return _name; }
+			void          affinity(unsigned) { }
 	};
 }