Rename TARGET_OS_* to HOST_OS_*.

runtime/vm/virtual_memory_win.cc

Index: runtime/vm/virtual_memory_win.cc
diff --git a/runtime/vm/virtual_memory_win.cc b/runtime/vm/virtual_memory_win.cc
index 0f4b1797d60257f5de79faa69f21e9dc5f92c072..c02f93a85645b5bd6a20d13919c5610d1b698fc5 100644
--- a/runtime/vm/virtual_memory_win.cc
+++ b/runtime/vm/virtual_memory_win.cc
@@ -3,7 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/globals.h"
-#if defined(TARGET_OS_WINDOWS)
+#if defined(HOST_OS_WINDOWS)
 
 #include "vm/virtual_memory.h"
 
@@ -98,4 +98,4 @@ bool VirtualMemory::Protect(void* address, intptr_t size, Protection mode) {
 
 }  // namespace dart
 
-#endif  // defined(TARGET_OS_WINDOWS)
+#endif  // defined(HOST_OS_WINDOWS)