Vendored mimalloc references macOS preprocessor symbols that no longer exist

Bug report

Bug description:

The CPython sources include a vendored copy of https://github.com/microsoft/mimalloc.

This was added 5 months ago as part of #109914. the commit comment says it is version "v2.12". That doesn't appear to be a mimalloc release, but v2.1.2 does exist, and was the current stable release as of November 2023.

However, that code references TARGET_IOS_IPHONE and TARGET_IOS_SIMULATOR. These symbols were deprecated in iOS 15 (released ~2 years ago). These references have been updated in the dev branch of mimalloc (along with many other changes), but there isn't an updated stable mimalloc release at time of writing.

We have also made a number of other changes to the vendored version (about 18 commits, by my count). It's not clear if these mirror changes that have been made upstream, or if we have effectively forked mimalloc at this point.

The code still appears to work at present, but there's a risk it might not in future when the symbol deprecations are finalised. There may also be other updates and bugfixes in mimalloc that we're not taking advantage of.

This was discovered during an audit of TARGET_OS symbol usage following the report of #117886 and #117891.

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Beginne in den vendorten mimalloc-Quellen von CPython mit der Suche nach TARGET_IOS_IPHONE und TARGET_IOS_SIMULATOR und vergleiche diese Kopie anschließend mit dem Entwicklungszweig von mimalloc und den ungefähr 18 lokalen Commits. Als abgeschlossen gilt die Dokumentation dazu, ob die deprecated-Symbole und andere relevante Upstream-Fixes übernommen werden sollten oder ob die vendorte Kopie zu einem gepflegten Fork abgewichen ist.