Without pointer compression, the max string length on 64-bit platforms
used to be 2**30 (minus header). With pointer-compression, this was
accidentally lowered to 2**28 (which is the historical limit for 32-bit
platforms). This CL bumps the limit on 64-bit platforms to 2**29, which
is the maximum we can support given that any heap object's size in bytes
must fit into a Smi (which are now 31-bit on all 64-bit platforms, with
or without pointer compression).

Change-Id: I263544317d9e6137f6b6a044784a21f41a2761b0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2030916Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66083}