This adds a kRuntimeCallArgvRegister definition in platform-specific
macro assemblers and unifies interface descriptor initialization.

Bug: v8:6666
Change-Id: I6fd98bf9ac46083dd2e557ab558d0a5117dd7b94
Reviewed-on: https://chromium-review.googlesource.com/1185003Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55310}