Add OS detection for iOS builds. If we are building for an iOS target,
the following V8 OS defines will be set:

V8_OS_BSD
V8_OS_MACOSX
V8_OS_POSIX
V8_OS_IOS  // This one is new.

The detection code is taken from Chromium's build_config.h file.

Bug: v8:9140
Change-Id: I435a8931dc0ae0eefdb893bc838a04470bcc57db
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1569435Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60897}