Previously, V8_OS_MACOSX was, somewhat confusingly, also used for iOS.
With this CL, V8_OS_DARWIN will be set on both macOS and iOS,
V8_OS_MACOS only on macOS, and V8_OS_IOS only on iOS.

This CL also renames V8_TARGET_OS_MACOSX to V8_TARGET_OS_MACOS and
renames platform-xnu.cc to platform-darwin.cc.

Change-Id: I4bcafc7c337586662114144f6c7ccf47d978da1f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3468577Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Samuel Groß <saelo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79167}