perf: Reorder & optimize perf_event_context to remove alignment padding on 64 bit builds

Remove 8 bytes of alignment padding from perf_event_context on 64 bit
builds which shrinks its size to 192 bytes allowing it to fit into one
fewer cache lines and into a smaller slab.

Signed-off-by: Richard Kennedy <richard@rsk.demon.co.uk>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1299512819.2039.5.camel@castor.rsk>
Signed-off-by: Ingo Molnar <mingo@elte.hu>