message: Ruy: Add mechanism to mask out paths.
  For example, a "superior" path like can be disabled so that an "inferior" path like AVX2 is executed on a more capable machine.
diff: @@ -40,6 +40,13 @@ Path Context::GetRuntimeEnabledPaths() {
  // Need to resolve now. Start by considering all paths enabled.
  runtime_enabled_paths_ = kAllPaths;
  + // This mechanism is intended to be used for testing and benchmarking. For
  + // example, one can set RUY_FORCE_DISABLE_PATHS to Path::kAvx512 in order to
  + // evaluate AVX2 performance on an AVX-512 machine.
  +#ifdef RUY_FORCE_DISABLE_PATHS
  + runtime_enabled_paths_ = runtime_enabled_paths_ & ~(RUY_FORCE_DISABLE_PATHS);
  +#endif
  +
  #if RUY_PLATFORM(ARM)
  // Now selectively disable paths that aren't supported on this machine.
  if ((runtime_enabled_paths_ & Path::kNeonDotprod) != Path::kNone) {
  