Kernel+LibC: Add a dump_backtrace() syscall.

This is very simple but already very useful. Now you're able to call to
dump_backtrace() from anywhere userspace to get a nice symbolicated
backtrace in the debugger output. :^)

Kernel/Process.h

@@ -104,6 +104,7 @@ public:
     void die();
     void finalize();
 
+    int sys$dump_backtrace();
     int sys$gettid();
     int sys$donate(int tid);
     int sys$shm_open(const char* name, int flags, mode_t);