Kernel: Implement a simple process time profiler

The kernel now supports basic profiling of all the threads in a process
by calling profiling_enable(pid_t). You finish the profiling by calling
profiling_disable(pid_t).

This all works by recording thread stacks when the timer interrupt
fires and the current thread is in a process being profiled.
Note that symbolication is deferred until profiling_disable() to avoid
adding more noise than necessary to the profile.

A simple "/bin/profile" command is included here that can be used to
start/stop profiling like so:

    $ profile 10 on
    ... wait ...
    $ profile 10 off

After a profile has been recorded, it can be fetched in /proc/profile

There are various limits (or "bugs") on this mechanism at the moment:

- Only one process can be profiled at a time.
- We allocate 8MB for the samples, if you use more space, things will
  not work, and probably break a bit.
- Things will probably fall apart if the profiled process dies during
  profiling, or while extracing /proc/profile

Libraries/LibC/serenity.h

@@ -41,4 +41,7 @@ __BEGIN_DECLS
 int module_load(const char* path, size_t path_length);
 int module_unload(const char* name, size_t name_length);
 
+int profiling_enable(pid_t);
+int profiling_disable(pid_t);
+
 __END_DECLS