LibC+Kernel: Start implementing sysconf

For now, only the non-standard _SC_NPROCESSORS_CONF and
_SC_NPROCESSORS_ONLN are implemented.

Use them to make ninja pick a better default -j value.
While here, make the ninja package script not fail if
no other port has been built yet.

Kernel/Arch/i386/CPU.h

@@ -744,6 +744,8 @@ public:
 
     static Processor& by_id(u32 cpu);
 
+    static size_t processor_count() { return processors().size(); }
+
     template<typename Callback>
     static inline IterationDecision for_each(Callback callback)
     {