Add more missing libc/libc++ functions

- Add sched_rr_get_interval()
- Add `unbuffer` command example
- Add more locale function stubs
- Vendor most of remaining libcxx content

third_party/libcxx/iostream

@@ -0,0 +1,64 @@
+// -*- C++ -*-
+// clang-format off
+//===--------------------------- iostream ---------------------------------===//
+//
+// 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 _LIBCPP_IOSTREAM
+#define _LIBCPP_IOSTREAM
+
+/*
+    iostream synopsis
+
+#include <ios>
+#include <streambuf>
+#include <istream>
+#include <ostream>
+
+namespace std {
+
+extern istream cin;
+extern ostream cout;
+extern ostream cerr;
+extern ostream clog;
+extern wistream wcin;
+extern wostream wcout;
+extern wostream wcerr;
+extern wostream wclog;
+
+}  // std
+
+*/
+
+#include "third_party/libcxx/__config"
+#include "third_party/libcxx/ios"
+#include "third_party/libcxx/streambuf"
+#include "third_party/libcxx/istream"
+#include "third_party/libcxx/ostream"
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#ifndef _LIBCPP_HAS_NO_STDIN
+extern _LIBCPP_FUNC_VIS istream cin;
+extern _LIBCPP_FUNC_VIS wistream wcin;
+#endif
+#ifndef _LIBCPP_HAS_NO_STDOUT
+extern _LIBCPP_FUNC_VIS ostream cout;
+extern _LIBCPP_FUNC_VIS wostream wcout;
+#endif
+extern _LIBCPP_FUNC_VIS ostream cerr;
+extern _LIBCPP_FUNC_VIS wostream wcerr;
+extern _LIBCPP_FUNC_VIS ostream clog;
+extern _LIBCPP_FUNC_VIS wostream wclog;
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif  // _LIBCPP_IOSTREAM