The new shared memory API should only be used on macOS, but
platform-macos.cc was also included on iOS, causing build failures. This
CL splits platform-macos.cc into platform-xnu.cc (common code for macOS
and iOS) and platform-macos.cc (the macOS specific parts)

Bug: chromium:1218005
Change-Id: Iab332865ffd8990ddd246bb9c08802909464d7e6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3468895Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Samuel Groß <saelo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79131}