Add __attribute__((const)) so that the compiler may assume that the
variable doesn't change across calls (basically treat the variable as
const).

Bug: chromium:1325007
Change-Id: I1ecd9a6b7b142cbb9da20a44f568465e1ca45fe7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3688400
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81043}