hrtimer: reorder struct hrtimer to save 8 bytes on 64bit builds

reorder struct hrtimer to save 8 bytes on 64 bit builds when
CONFIG_TIMER_STATS selected.  (also removes 8 bytes from signal_struct)

Signed-off-by: Richard Kennedy <richard@rsk.demon.co.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>