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-codezero/src/core/include/platform_thread.h

@@ -114,9 +114,9 @@ namespace Genode {
 			unsigned long pager_object_badge() const { return _tid; }
 
 			/**
-			 * Set the executing CPU for this thread.
+			 * Set the executing CPU for this thread
 			 */
-			void set_cpu(unsigned int cpu_no);
+			void affinity(unsigned cpu);
 
 			/**
 			 * Get thread name