[llvm-exegesis] Add support for validation counters (#76653)

This patch adds support for validation counters. Validation counters can
be used to measure events that occur during snippet execution like cache
misses to ensure that certain assumed invariants about the benchmark
actually hold. Validation counters are setup within a perf event group,
so are turned on and off at exactly the same time as the "group leader"
counter that measures the desired value.