The CL splits the Oilpan giga-cage in two 2GB reservations: one for
normal pages and the other for large ones. The split enables fast
page-header lookup (assuming most objects reside on normal pages), which
is needed for:
1) the young generation project, where the remembered set will move to
   pages;
2) the shared-cage project, to find HeapBase* from page-headers.

Bug: v8:12231, chromium:1029379
Change-Id: I4ae9e8a75a307ed0dff9a2ec4f1247b80e17ebd9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3688519
Auto-Submit: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80950}