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/experimental/__config

@@ -0,0 +1,80 @@
+// -*- C++ -*-
+// clang-format off
+//===--------------------------- __config ---------------------------------===//
+//
+// 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_EXPERIMENTAL_CONFIG
+#define _LIBCPP_EXPERIMENTAL_CONFIG
+
+#include "third_party/libcxx/__config"
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+#define _LIBCPP_BEGIN_NAMESPACE_EXPERIMENTAL namespace std { namespace experimental {
+#define _LIBCPP_END_NAMESPACE_EXPERIMENTAL  } }
+#define _VSTD_EXPERIMENTAL std::experimental
+
+#define _LIBCPP_BEGIN_NAMESPACE_LFTS _LIBCPP_BEGIN_NAMESPACE_EXPERIMENTAL inline namespace fundamentals_v1 {
+#define _LIBCPP_END_NAMESPACE_LFTS  } } }
+#define _VSTD_LFTS _VSTD_EXPERIMENTAL::fundamentals_v1
+
+#define _LIBCPP_BEGIN_NAMESPACE_LFTS_V2 _LIBCPP_BEGIN_NAMESPACE_EXPERIMENTAL inline namespace fundamentals_v2 {
+#define _LIBCPP_END_NAMESPACE_LFTS_V2  } } }
+#define _VSTD_LFTS_V2 _VSTD_EXPERIMENTAL::fundamentals_v2
+
+#define _LIBCPP_BEGIN_NAMESPACE_LFTS_PMR _LIBCPP_BEGIN_NAMESPACE_LFTS namespace pmr {
+#define _LIBCPP_END_NAMESPACE_LFTS_PMR _LIBCPP_END_NAMESPACE_LFTS }
+#define _VSTD_LFTS_PMR _VSTD_LFTS::pmr
+
+#define _LIBCPP_BEGIN_NAMESPACE_CHRONO_LFTS _LIBCPP_BEGIN_NAMESPACE_STD        \
+  namespace chrono { namespace experimental { inline namespace fundamentals_v1 {
+#define _LIBCPP_END_NAMESPACE_CHRONO_LFTS _LIBCPP_END_NAMESPACE_STD } } }
+
+#if defined(_LIBCPP_NO_EXPERIMENTAL_DEPRECATION_WARNING_FILESYSTEM)
+#   define _LIBCPP_DEPRECATED_EXPERIMENTAL_FILESYSTEM /* nothing */
+#else
+#   define _LIBCPP_DEPRECATED_EXPERIMENTAL_FILESYSTEM __attribute__((deprecated("std::experimental::filesystem has now been deprecated in favor of C++17's std::filesystem. Please stop using it and start using std::filesystem. This experimental version will be removed in LLVM 11. You can remove this warning by defining the _LIBCPP_NO_EXPERIMENTAL_DEPRECATION_WARNING_FILESYSTEM macro.")))
+#endif
+
+#define _LIBCPP_BEGIN_NAMESPACE_EXPERIMENTAL_FILESYSTEM \
+    _LIBCPP_BEGIN_NAMESPACE_EXPERIMENTAL namespace filesystem _LIBCPP_DEPRECATED_EXPERIMENTAL_FILESYSTEM { \
+    inline namespace v1 {
+
+#define _LIBCPP_END_NAMESPACE_EXPERIMENTAL_FILESYSTEM \
+    } } _LIBCPP_END_NAMESPACE_EXPERIMENTAL
+
+#define _LIBCPP_BEGIN_NAMESPACE_EXPERIMENTAL_COROUTINES \
+  _LIBCPP_BEGIN_NAMESPACE_EXPERIMENTAL inline namespace coroutines_v1 {
+
+#define _LIBCPP_END_NAMESPACE_EXPERIMENTAL_COROUTINES \
+  } _LIBCPP_END_NAMESPACE_EXPERIMENTAL
+
+#define _VSTD_CORO _VSTD_EXPERIMENTAL::coroutines_v1
+
+#define _LIBCPP_BEGIN_NAMESPACE_EXPERIMENTAL_SIMD \
+    _LIBCPP_BEGIN_NAMESPACE_EXPERIMENTAL inline namespace parallelism_v2 {
+
+#define _LIBCPP_END_NAMESPACE_EXPERIMENTAL_SIMD \
+    } _LIBCPP_END_NAMESPACE_EXPERIMENTAL
+
+#define _LIBCPP_BEGIN_NAMESPACE_EXPERIMENTAL_SIMD_ABI \
+    _LIBCPP_BEGIN_NAMESPACE_EXPERIMENTAL_SIMD namespace simd_abi {
+
+#define _LIBCPP_END_NAMESPACE_EXPERIMENTAL_SIMD_ABI \
+    } _LIBCPP_END_NAMESPACE_EXPERIMENTAL_SIMD
+
+// TODO: support more targets
+#if defined(__AVX__)
+#define _LIBCPP_NATIVE_SIMD_WIDTH_IN_BYTES 32
+#else
+#define _LIBCPP_NATIVE_SIMD_WIDTH_IN_BYTES 16
+#endif
+
+#endif