Optimize control flow in the termination phase of the exception handler

* Ensure that the sequential code path from ret_from_exception to
  restore_all aligns with the expected common case of resuming userspace
  without supplementary work.  Separate the ret_from_fork and
  ret_from_kernel_thread stubs from the handle_exception body.
* Disable interrupts when checking the thread_info flags, but re-enable
  them prior to calling do_notify_resume().
* Simplify the asm/thread_info.h header.
* Revert to the generic (empty) asm/segment.h header, as customization
  is useful only for x86.
Use generic idle loop

* Remove deprecated cpu_idle() function.
	"idle: Consolidate idle implementations" [0]
	"idle: Provide a generic entry point for the idle code"
		a1a04ec3c7c27a682473fd9beb2c996316a64649
	"idle: Implement generic idle function"
		d166991234347215dc23fc9dc15a63a83a1a54e1
* Reorganize the asm/processor.h header for greater clarity.

References:
	[0] <http://thread.gmane.org/gmane.linux.kernel/1461723>
Enhance stack tracing infrastructure

* Replace show_backtrace() with a reusable unwind iterator framework.
* Resort to alternate raw backtracing if CONFIG_FRAME_POINTER is unset.
* Implement save_stack_trace() to enable CONFIG_STACKTRACE_SUPPORT, upon
  which several kernel function tracers and debugging features depend.
* Adopt generic dump_stack() implementation.
	"dump_stack: consolidate dump_stack() implementations and unify
	 their behaviors"
		196779b9b4ce1922afabdc20d0270720603bd46c
* Ignore scheduler-related functions in get_wchan().
* Provide a basic definition for show_regs().