Fix platform availability of some os/posix constants (#10831)

tests/stubtest_allowlists/darwin.txt

@@ -1,12 +1,10 @@
 _?curses.A_ITALIC
 
 _posixsubprocess.cloexec_pipe
-os.EX_NOTFOUND
 os.SF_MNOWAIT
 os.SF_NODISKIO
 os.SF_SYNC
 (os|posix).sched_param  # system dependent. Unclear if macos has it.
-posix.EX_NOTFOUND
 select.KQ_FILTER_NETDEV  # system dependent
 select.kqueue.__init__  # default C signature is wrong
 select.POLLMSG   # system dependent
@@ -33,7 +31,6 @@ _ctypes.dlclose
 _ctypes.dlopen
 _ctypes.dlsym
 
-posix.NGROUPS_MAX
 webbrowser.MacOSX.__init__
 
 # ==========

tests/stubtest_allowlists/linux.txt

@@ -1,12 +1,9 @@
 _socket.*
 _posixsubprocess.cloexec_pipe
-os.EX_NOTFOUND
 os.SF_MNOWAIT
 os.SF_NODISKIO
 os.SF_SYNC
 os.plock
-posix.EX_NOTFOUND
-posix.NGROUPS_MAX
 select.EPOLL_RDHUP
 selectors.KqueueSelector
 signal.SIGEMT
@@ -48,6 +45,10 @@ fcntl.I_[A-Z0-9_]+
 os.SCHED_[A-Z_]+
 posix.SCHED_[A-Z_]+
 
+# Exists on some Unix platforms, but not in CI:
+os.EX_NOTFOUND
+posix.EX_NOTFOUND
+
 # Some of these exist on non-windows, but they are useless and this is not intended
 stat.FILE_ATTRIBUTE_[A-Z_]+