We are not shipping ptr-compr in M79 on x64 because chromium:1009439
blocks 31-bit Smis on 64-bit architectures, so these's no point in
disabling double fields unboxing.

This CL will be reverted after the M79 branch point.

Bug: v8:9799, chromium:1009439
Change-Id: I28d0013d3ab06ce41d5028ba4f66c9b249de52d7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1862556Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64288}