Rename TARGET_OS_* to HOST_OS_*.

runtime/bin/thread.h

Index: runtime/bin/thread.h
diff --git a/runtime/bin/thread.h b/runtime/bin/thread.h
index 5bf1f13a008dadc5bdd0025f9bb5f441b7f80b2c..6b804441abbf13e14b4946bd8ec95c81f58c87d3 100644
--- a/runtime/bin/thread.h
+++ b/runtime/bin/thread.h
@@ -16,15 +16,15 @@ class Monitor;
 }
 
 // Declare the OS-specific types ahead of defining the generic classes.
-#if defined(TARGET_OS_ANDROID)
+#if defined(HOST_OS_ANDROID)
 #include "bin/thread_android.h"
-#elif defined(TARGET_OS_FUCHSIA)
+#elif defined(HOST_OS_FUCHSIA)
 #include "bin/thread_fuchsia.h"
-#elif defined(TARGET_OS_LINUX)
+#elif defined(HOST_OS_LINUX)
 #include "bin/thread_linux.h"
-#elif defined(TARGET_OS_MACOS)
+#elif defined(HOST_OS_MACOS)
 #include "bin/thread_macos.h"
-#elif defined(TARGET_OS_WINDOWS)
+#elif defined(HOST_OS_WINDOWS)
 #include "bin/thread_win.h"
 #else
 #error Unknown target os.