Kernel: Dump backtrace to debugger for DefaultSignalAction::DumpCore.

This makes assertion failures generate backtraces again. Sorry to everyone
who suffered from the lack of backtraces lately. :^)

We share code with the /proc/PID/stack implementation. You can now get the
current backtrace for a Thread via Thread::backtrace(), and all the traces
for a Process via Process::backtrace().

Kernel/Process.h

@@ -56,6 +56,8 @@ public:
         Ring3 = 3,
     };
 
+    String backtrace(ProcessInspectionHandle&) const;
+
     bool is_dead() const { return m_dead; }
 
     Thread::State state() const { return main_thread().state(); }