9bf9453e26
Links in an implementation of _availability_version_check on macOS.
This is required due to an upstream compiler builtin (runtime) change
that marked this function as weakly-linked via `__attribute__((weak import))`
As such, no linking failure occurs when the function is unavailable at
link time.
By providing an implementation, the linker picks up our implementation,
which looks up symbol in question at runtime (via dlsym) on the first
invocation, caches it for later invocations, then invokes it. This is,
in fact, precisely what the original clang builtin implementation did.
Upstream clang change: https://reviews.llvm.org/D150397
Issue: https://github.com/flutter/flutter/issues/133777
[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style