Moved DEFINE_UINT after the #ifdef block, so it is also defined
for Mac arm64.

This is a reland of 6aadf0e0

Original change's description:
> [baseline] Add max threads flags for CSP
>
> Bug: v8:12054
> Change-Id: I49238fe673858b60c2a2a935ed1f45f465269216
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3308707
> Auto-Submit: Victor Gomes <victorgomes@chromium.org>
> Commit-Queue: Camillo Bruni <cbruni@chromium.org>
> Reviewed-by: Camillo Bruni <cbruni@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#78187}

Bug: v8:12054
Change-Id: Ica352b2f099fff875411c0e1915bcddb4ca48061
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3310800Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Victor Gomes <victorgomes@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78192}