add missing _ctypes functions (#13067)

stdlib/@tests/stubtest_allowlists/darwin.txt

@@ -24,11 +24,6 @@ select.POLLMSG   # system dependent
 (posix.ST_[A-Z]+)?
 (termios.[A-Z0-9_]+)?
 
-# Exists at runtime, but missing from stubs
-_ctypes.dlclose
-_ctypes.dlopen
-_ctypes.dlsym
-
 # ==========
 # Allowlist entries that cannot or should not be fixed
 # ==========

stdlib/@tests/stubtest_allowlists/linux.txt

@@ -5,10 +5,6 @@
 curses.has_key
 selectors.KqueueSelector
 
-_ctypes.dlclose
-_ctypes.dlopen
-_ctypes.dlsym
-
 # ==========
 # Allowlist entries that cannot or should not be fixed
 # ==========

stdlib/_ctypes.pyi

@@ -1,6 +1,6 @@
 import _typeshed
 import sys
-from _typeshed import ReadableBuffer, WriteableBuffer
+from _typeshed import ReadableBuffer, StrOrBytesPath, WriteableBuffer
 from abc import abstractmethod
 from collections.abc import Callable, Iterable, Iterator, Mapping, Sequence
 from ctypes import CDLL, ArgumentError as ArgumentError, c_void_p
@@ -48,6 +48,12 @@ if sys.platform == "win32":
     def LoadLibrary(name: str, load_flags: int = 0, /) -> int: ...
     def FreeLibrary(handle: int, /) -> None: ...
 
+else:
+    def dlclose(handle: int, /) -> None: ...
+    # The default for flag is RTLD_GLOBAL|RTLD_LOCAL, which is platform dependent.
+    def dlopen(name: StrOrBytesPath, flag: int = ..., /) -> int: ...
+    def dlsym(handle: int, name: str, /) -> int: ...
+
 if sys.version_info >= (3, 13):
     # This class is not exposed. It calls itself _ctypes.CType_Type.
     @type_check_only