[Cygwin] CYGWIN is not WIN32 in current CMake

On old CMake, Cygwin were also WIN32 but currently not.
LLVM_ON_UNIX=1 and LLVM_HAVE_LINK_VERSION_SCRIPT=0 should
be defined for Cygwin target.

Author: kikairoya

llvm/cmake/modules/HandleLLVMOptions.cmake

@@ -214,15 +214,15 @@ if( LLVM_REVERSE_ITERATION )
   set( LLVM_ENABLE_REVERSE_ITERATION 1 )
 endif()
 
-if(WIN32)
+if(WIN32 OR CYGWIN)
   set(LLVM_HAVE_LINK_VERSION_SCRIPT 0)
   if(CYGWIN)
     set(LLVM_ON_WIN32 0)
     set(LLVM_ON_UNIX 1)
-  else(CYGWIN)
+  else()
     set(LLVM_ON_WIN32 1)
     set(LLVM_ON_UNIX 0)
-  endif(CYGWIN)
+  endif()
 elseif(FUCHSIA OR UNIX)
   set(LLVM_ON_WIN32 0)
   set(LLVM_ON_UNIX 1)