Background: the first page of each space is implicitly immovable.
Recently, our builtin code objects have reached a size at which we
fill up the first page of code space during initialization. Once
that occurs, newly requested allocations of immovable code are
allocated in a large object space page of 512K.

This CL mitigates these effects by simply marking pages as immovable
during snapshot creation instead of going into LO space.

On snapshot builds, this should just work: deserialized pages are
trimmed and marked immovable when deserialization finishes.

However, non-snapshot builds and allocations of immovable CEntryStub
code at runtime are still affected.

BUG=v8:5831

Review-Url: https://codereview.chromium.org/2635973002
Cr-Commit-Position: refs/heads/master@{#42411}