[turboshaft] stage behind --future

Bug: v8:12783
Change-Id: I723438d4843861b5933f1ea1f649ae426a2a1c04
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3899125
Auto-Submit: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Darius Mercadier <dmercadier@chromium.org>
Reviewed-by: Darius Mercadier <dmercadier@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83266}

src/flags/flag-definitions.h

@@ -959,6 +959,7 @@ DEFINE_FLOAT(script_delay_fraction, 0.0,
              "run's duration")
 
 DEFINE_BOOL(turboshaft, false, "enable TurboFan's Turboshaft phases")
+DEFINE_WEAK_IMPLICATION(future, turboshaft)
 DEFINE_BOOL(turboshaft_trace_reduction, false,
             "trace individual Turboshaft reduction steps")