Kernel: Return the actual number of CPU cores that we have

... instead of returning the maximum number of Processor objects that we
can allocate.

Some ports (e.g. gdb) rely on this information to determine the number
of worker threads to spawn. When gdb spawned 64 threads, the kernel
could not cope with generating backtraces for it, which prevented us
from debugging it properly.

This commit also removes the confusingly named
`Processor::processor_count` function so that this mistake can't happen
again.

Kernel/Arch/x86/Processor.h

@@ -179,8 +179,6 @@ public:
     void flush_gdt();
     const DescriptorTablePointer& get_gdtr();
 
-    static size_t processor_count() { return processors().size(); }
-
     template<IteratorFunction<Processor&> Callback>
     static inline IterationDecision for_each(Callback callback)
     {