This is more consistent with similar features, for example
V8_ENABLE_WEBASSEMBLY or V8_ENABLE_MAGLEV.

Drive-by: remove V8_SANDBOX_IS_AVAILABLE as it's no longer needed.

Bug: v8:10391
Change-Id: I8658c5b0c331a4c73892737083b2c2f9b8f84056
Cq-Include-Trybots: luci.v8.try:v8_linux64_heap_sandbox_dbg_ng,v8_linux_arm64_sim_heap_sandbox_dbg_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3647355
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Auto-Submit: Samuel Groß <saelo@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80530}