[arm64] Don't use x18 (platform register) in assembler tests

This fixes the assembler tests on Windows arm64, that were previously crashing.

Bug: v8:9235
Change-Id: I616e6eb00bb71d70195787f80f8b54a9ae33abe6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1634919
Commit-Queue: Martyn Capewell <martyn.capewell@arm.com>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61981}