gh-156112: Load the ObjC runtime by name when find_library() cannot resolve it

[clementperon]

_ios_support raises ImportError at import when ctypes.util.find_library("objc") returns None. That happens whenever the dyld shared cache cannot be queried: dyld_find() falls back to _dyld_shared_cache_contains_path(), which Modules/_ctypes/callproc.c guards with __builtin_available(macOS 11.0, iOS 14.0, tvOS 14.0, watchOS 7.0, *), so on iOS 13 it raises NotImplementedError and no path can be resolved for a library that exists only in the cache.

IPHONEOS_DEPLOYMENT_TARGET defaults to 13.0, so this sits inside the supported range, and it takes platform.platform() and platform.ios_ver() down with it.

dyld resolves a bare library name against the cache by itself, so this falls back to the name when find_library() comes up empty, and keeps reporting a real load failure as ImportError rather than letting OSError escape.

Verified on macOS, where the same cache mechanism applies — find_library("objc") names a path that is not a file, and a bare-name load works:

>>> util.find_library("objc"), os.path.isfile("/usr/lib/libobjc.dylib")
('/usr/lib/libobjc.dylib', False)
>>> cdll.LoadLibrary("libobjc.dylib")
<CDLL 'libobjc.dylib', handle ... at ...>

and with find_library patched to return None, the module loads the runtime and objc_getClass(b"NSProcessInfo") succeeds.

I do not have an iOS 13 device or simulator runtime available, so the import failure itself is derived from the source rather than reproduced on-device; the fallback is verified as above.

• Issue: _ios_support fails to import on iOS 13, where find_library() cannot query the dyld shared cache #156112