syscall latency improvement

 * There's now an asm/thread_info.h header file with the basic structure
   def and asm offsets in it.

 * There's now a linux/thread_info.h header file which includes the asm
   version and wraps some bitops calls to make convenience functions for
   accessing the low-level flags. 

 * The task_struct has had some fields removed (and some flags), and has
   acquired a pointer to the thread_info struct.

 * task_struct's are now allocated on slabs in kernel/fork.c, whereas
   thread_info structs are allocated at the bottom of the stack pages.

 * Some more convenience functions are provided at the end of linux/sched.h to
   access flags in other tasks (these are here because they need to access the
   task_struct).