perf/x86: Add compiler barrier after updating BTS

Since BTS is coherent, simply add a compiler barrier to separate the BTS
update and aux_head store.

Signed-off-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210809111407.596077-5-leo.yan@linaro.org