Exports are properties in the global object. Pre-reserve the space,
since we know the count upfront.

Bug: v8:11525
Change-Id: Ia8ea992234ed8cf71a1060254766b0ba31562436
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3416231Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78767}