[libc] pthread.h.def: add PTHREAD_MUTEX_INITIALIZER

[alfredfo]

Summary

Adds PTHREAD_MUTEX_INITIALIZER for compiler-rt's builtins/emutls.c

libc/include/pthread.h.def

 //===-- POSIX header pthread.h --------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_LIBC_PTHREAD_H
 #define LLVM_LIBC_PTHREAD_H
 
 #include <__llvm-libc-common.h>
 
 #define PTHREAD_STACK_MIN (1 << 14) // 16KB
 
+#define PTHREAD_MUTEX_INITIALIZER {0}
 #define PTHREAD_ONCE_INIT {0}
 
 enum {
   PTHREAD_CREATE_JOINABLE = 0x0,
   PTHREAD_CREATE_DETACHED = 0x1,
 
   PTHREAD_MUTEX_NORMAL = 0x0,
   PTHREAD_MUTEX_ERRORCHECK = 0x1,