[flang][CMake] CYGWIN: Fix undefined references at link time. (#67105)

While building the source of flang on CYGWIN, the process suddenly
stopped with lot of "Undefined reference" errors at link time.
According to the statement that a shared library can't have undefined
references on Windows, I applied the same fix to CYGWIN with this patch.

Author: carlo-bramini

flang/test/CMakeLists.txt

@@ -65,6 +65,7 @@ set(FLANG_TEST_DEPENDS
   bbc
   FortranDecimal
 )
+
 if (NOT FLANG_STANDALONE_BUILD)
   list(APPEND FLANG_TEST_DEPENDS
     llvm-config
@@ -79,7 +80,7 @@ if (NOT FLANG_STANDALONE_BUILD)
   )
 endif ()
 
-if (LLVM_BUILD_EXAMPLES AND LLVM_ENABLE_PLUGINS AND NOT WIN32 AND NOT FLANG_STANDALONE_BUILD)
+if (LLVM_BUILD_EXAMPLES AND LLVM_ENABLE_PLUGINS AND NOT (WIN32 OR CYGWIN) AND NOT FLANG_STANDALONE_BUILD)
   list(APPEND FLANG_TEST_DEPENDS Bye)
 endif()