Rename TARGET_OS_* to HOST_OS_*.

root_certificates.cc

Index: root_certificates.cc
diff --git a/root_certificates.cc b/root_certificates.cc
index a219d15e01b390f6e4efc2ceab033fdcbde8a077..93284913623a871f65932390bbcf14cf7a7ac58e 100644
--- a/root_certificates.cc
+++ b/root_certificates.cc
@@ -9,7 +9,7 @@
 #if !defined(DART_IO_DISABLED) && !defined(DART_IO_SECURE_SOCKET_DISABLED)
 
 #include "platform/globals.h"
-#if !defined(TARGET_OS_MACOS) && !defined(TARGET_OS_ANDROID)
+#if !defined(HOST_OS_MACOS) && !defined(HOST_OS_ANDROID)
 
 namespace dart {
 namespace bin {
@@ -20753,7 +20753,7 @@ unsigned int root_certificates_pem_length = 248749;
 }  // namespace bin
 }  // namespace dart
 
-#endif  // !defined(TARGET_OS_MACOS) && !defined(TARGET_OS_ANDROID)
+#endif  // !defined(HOST_OS_MACOS) && !defined(HOST_OS_ANDROID)
 
 #endif  // !defined(DART_IO_DISABLED) &&
         // !defined(DART_IO_SECURE_SOCKET_DISABLED)