Use rr-safe `nopl; rdtsc` sequence (#50975)
When running under `rr`, it needs to patch out `rdtsc` to record the
values returned. If this is not possible, `rr` falls back to an
expensive signal-based emulation. As of rr master, a specific `nopl;
rdtsc` sequence may be used to guarantee that `rdtsc` patching is always
possible. Use this sequence for uses of rdtsc in our runtime.
