This test allocates a large mapping and splits into kThunkBufferSize
areas that it needs to be able to change permissions on. So
kThunkBufferSize needs to be set to the largest page size possible,
which is 64k at the moment.

It doesn't matter if kThunkBufferSize is larger than the actual page
size.

Bug: v8:10808
Change-Id: I3a8947f04a7ec25be49a54015cd128e901065ea6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2463404Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Pierre Langlois <pierre.langlois@arm.com>
Cr-Commit-Position: refs/heads/master@{#70449}