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

从 CPython 中 vendored 的 mimalloc 源码开始，搜索 TARGET_IOS_IPHONE 和 TARGET_IOS_SIMULATOR，然后将该副本与 mimalloc 的开发分支以及大约 18 个本地 commit 进行比较。完成的标准是记录 deprecated 符号和其他相关的 upstream 修复是否应当纳入，或者记录 vendored 副本是否已经分歧为一个受维护的 fork。