Rename TARGET_OS_* to HOST_OS_*.

runtime/vm/virtual_memory_linux.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/globals.h"
-#if defined(TARGET_OS_LINUX)
+#if defined(HOST_OS_LINUX)
 
 #include "vm/virtual_memory.h"
 
 #include <sys/mman.h>  // NOLINT
 #include <unistd.h>    // NOLINT
 
 #include "platform/assert.h"
 #include "platform/utils.h"
 
 #include "vm/isolate.h"
@@ skipping 87 matching lines @@
       prot = PROT_READ | PROT_WRITE | PROT_EXEC;
       break;
   }
   return (mprotect(reinterpret_cast<void*>(page_address),
                    end_address - page_address, prot) == 0);
 }
 
 
 }  // namespace dart
 
-#endif  // defined(TARGET_OS_LINUX)
+#endif  // defined(HOST_OS_LINUX)