mac: Annotate proper availability for CTFontCopyVariationAxesInternal

CTFontCopyVariationAxesInternal is CoreText-internal function that does
not exist before macOS 12.1. Chrome doesn’t use it before macOS 13.0.

This became a problem in Chrome 140 due to the new use of chained fixups
(8da209e9fa12, https://chromium-review.googlesource.com/c/6729794,
2025-07-11), which were enabled in Chrome 140 as the first version that
requires macOS 12, as chained fixups themselves require macOS 12. This
is a load-time performance improvement, but it disables lazy-binding
semantics.

Even though Chrome didn’t use CTFontCopyVariationAxesInternal on macOS <
13, load-time binding means that all symbols must resolve at load time.
This caused a launch failure on macOS 12.0 and 12.0.1.

Annotating CTFontCopyVariationAxesInternal with an availability
attribute that properly specifies that it’s only available on macOS ≥
12.1 causes the symbol in linked output to be marked as weak, which
should have been the case all along. It’s valid for a weak symbol to be
missing at runtime, and dyld will resolve it to 0 (nullptr), even under
bind-at-load with chained fixups. Since Chrome never attempts to call
this function on macOS < 13, it will never be nullptr when used, so this
is safe.

Bug: 443764291
Change-Id: I7f1a99aea77415931e985384c05caeebde34ae1c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6927630
Commit-Queue: Mark Mentovai <mark@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Auto-Submit: Mark Mentovai <mark@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1512777}