[tsan] force tls_model='initial-exec' for tsan's cur_thread_placeholder so that we don't depend on the compiler flags

llvm-svn: 208536

compiler-rt/lib/tsan/rtl/tsan_rtl.h

@@ -473,6 +473,7 @@ struct ThreadState {
 };
 
 #ifndef TSAN_GO
+__attribute__((tls_model("initial-exec")))
 extern THREADLOCAL char cur_thread_placeholder[];
 INLINE ThreadState *cur_thread() {
   return reinterpret_cast<ThreadState *>(&cur_thread_placeholder);