perf thread: Save previous sample for LBR stitching approach

To retrieve the overwritten LBRs from previous sample for LBR stitching
approach, perf has to save the previous sample.

Only allocate the struct lbr_stitch once, when LBR stitching approach is
enabled and kernel supports hw_idx.

Signed-off-by: Kan Liang <[email protected]>
Reviewed-by: Andi Kleen <[email protected]>
Acked-by: Jiri Olsa <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Alexey Budankov <[email protected]>
Cc: Mathieu Poirier <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Pavel Gerasimov <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Ravi Bangoria <[email protected]>
Cc: Stephane Eranian <[email protected]>
Cc: Vitaly Slobodskoy <[email protected]>
Link: http://lore.kernel.org/lkml/[email protected]
[ Use zalloc()/zfree() for thread->lbr_stitch ]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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