BenchmarkWalltime measures the single CLOCK_REALTIME vDSO read the built-in
header uses against a plain time.Now().Unix(), which reads CLOCK_REALTIME and
CLOCK_MONOTONIC and then unpacks a time.Time.

arm64, go1.27, n=3:
  walltime         33.55-34.28 ns/op
  time.Now().Unix  71.54-71.65 ns/op