Add a reliable way to tell if per CPU or per thread caching is being used

When running on Centos7, which is on linux kernel 3.10 and thus shouldn't have RSEQ support, `MallocExtension::PerCpuCachesActive()` returns true, even though per-thread caches are being used (this can be verified by checking the sizes of the per-cpu and per-thread freelists in DumpStats). Either `PerCpuCachesActive` should return false here, or there should be some other way of checking whether per cpu caching or per thread caching is being used.