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}

The motivation is being able to build Chrome/Mac/Intel on an
Apple Silicon mac.

Depends on https://chromium-review.googlesource.com/c/chromium/src/+/3348020

- Correctly set v8_snapshot_toolchain when targeting x64 on an arm64
  host (always use the clang_ toolchain for now since that's all
  that's needed at the moment)

- Check V8_HOST_ARCH in immediate-crash.h. In V8 terminology, "host"
  is the machine the snapshot generation runs on, while "target" is the
  machine that V8 runs on when it JITs. IMMEDIATE_CRASH runs on the
  host. Up to now, target arch x64 implied host arch x64 so the old code
  happened to work too, but this is the correct macro (and it makes this
  cross scenario work).

- In assembler-x64.cc, only compile the code that probes the current CPU
  when running on an intel host. (There's an early return for snapshot
  generation anyways.)

Bug: chromium:1280968
Change-Id: I4821a5994de8ed5f9e4f62184dc6ab6f5223bc3f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3348040Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78417}

Release-official builds strip error messages from CHECK messages.
This can make it difficult to distinguish a CHECK crash location in
crash reports. As such, instead of using V8_FatalNoContext, import the
IMMEDIATE_CRASH macro from chromium and use that instead, which should
cause a crash directly in the instruction stream so that the top
stackframe on the crash report directly identifies the CHECK location
that failed.

More details here:
https://docs.google.com/document/d/1tyMwzxUNH8BctM_urSQIYdcbwmzP4kTnwEjnFamBpKY

Change-Id: I5b8175f19571834f790060d641db08d0b9c2c17b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2756223Reviewed-by: Mythri Alle <mythria@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73430}