Enable GC in thread_attachThis in scope(success) instead scope(exit)

Since thread_attachThis is nothrow, this is the same as scope(exit),
just it will avoid using GC when the Error is thrown, possibly by
the GC itself.

Author: Burgos

src/core/thread.d

@@ -2075,7 +2075,7 @@ extern (C) bool thread_isMainThread() nothrow @nogc
  */
 extern (C) Thread thread_attachThis() nothrow
 {
-    GC.disable(); scope(exit) GC.enable();
+    GC.disable(); scope(success) GC.enable();
 
     if (auto t = Thread.getThis())
         return t;