[LIT][Cygwin] Mark ulimit -f unsupported for Cygwin (#165849)

Cygwin doesn't support `ulimit -f` because Windows doesn't provide such
functionality.

Author: kikairoya

llvm/utils/lit/tests/shtest-ulimit-nondarwin.py

@@ -2,7 +2,7 @@
 
 # ulimit does not work on non-POSIX platforms.
 # These tests are specific to options that Darwin does not support.
-# UNSUPPORTED: system-windows, system-darwin, system-aix, system-solaris
+# UNSUPPORTED: system-windows, system-cygwin, system-darwin, system-aix, system-solaris
 
 # RUN: not %{lit} -a -v %{inputs}/shtest-ulimit-nondarwin | FileCheck %s