[libc] Surround get_start_args_addr in __llvm_libc namespace.

[PaulkaToast]

Summary

Caught by libc-tidy from patch D76818.

libc/src/threads/linux/x86_64/thread_start_args.h.in

 //===- x86_64 implementation of the get_start_args_addr function -*- 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
 //
 //===----------------------------------------------------------------------===//
 
 %%begin()
 
+namespace __llvm_libc {
+
 __attribute__((always_inline)) inline uintptr_t get_start_args_addr() {
   // NOTE: For __builtin_frame_address to work reliably across compilers,
   // architectures and various optimization levels, the TU including this file
   // should be compiled with -fno-omit-frame-pointer.
   return reinterpret_cast<uintptr_t>(__builtin_frame_address(0)) +
          // The x86_64 call instruction pushes resume address on to the stack.
          // Next, The x86_64 SysV ABI also pushes the frame pointer on the
          // stack. Hence, we look past these items to get to the start args.
          sizeof(uintptr_t) * 2;
 }
 
+} // namespace __llvm_libc