[libc] [NFC] Remove -Wc11-extensions warning

[abrachet]

Summary

_Atomic is not an official C++ keyword, when -pedantic is specified the compiler gives a warning about its usage.

This patch uses <stdatomic.h> and it's atomic_uint_least32_t. Unfortunately there is no atomic_uint32_t. This works because <stdatomic.h> is a system header which isn't subject to the same warnings. Alternatively we could add #pragma GCC system_header to thread_utils.h.

[abrachet]

It's worth mentioning compiler version I was getting this warning on.

$ clang --version
clang version 11.0.0 (https://github.com/llvm/llvm-project.git 6d2372ce584f0fd26575e31d63278b98be2cfa1c)

[sivachandra]

I will fix this separately as I am going to revert the original patch shortly. It is failing on the bots.

libc/src/threads/linux/thread_utils.h

 //===--- Linux specific definitions to support mutex operations --*- C++ -*===//
 //
 // 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_SRC_THREADS_LINUX_THREAD_UTILS_H
 #define LLVM_LIBC_SRC_THREADS_LINUX_THREAD_UTILS_H
 
+#include "include/threads.h"
+
+#include <stdatomic.h>
 #include <stdint.h>
 
-using FutexData = _Atomic uint32_t;
+using FutexData = atomic_uint_least32_t;
+static_assert(sizeof(FutexData) == sizeof(thrd_t::__clear_tid),
+              "FutexData must be the same size as thrd_t::__clear_tid");
 
 struct ThreadParams {
   static constexpr uintptr_t DefaultStackSize = 1 << 15; // 32 KB
   static constexpr uint32_t ClearTIDValue = 0xABCD1234;
 };
 
 #endif // LLVM_LIBC_SRC_THREADS_LINUX_THREAD_UTILS_H