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/include/config_elast.hh

@@ -0,0 +1,36 @@
+// clang-format off
+//===----------------------- config_elast.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 _LIBCPP_CONFIG_ELAST
+#define _LIBCPP_CONFIG_ELAST
+
+#include "third_party/libcxx/__config"
+
+#if defined(ELAST)
+#define _LIBCPP_ELAST ELAST
+#elif defined(_NEWLIB_VERSION)
+#define _LIBCPP_ELAST __ELASTERROR
+#elif defined(__Fuchsia__)
+// No _LIBCPP_ELAST needed on Fuchsia
+#elif defined(__wasi__)
+// No _LIBCPP_ELAST needed on WASI
+#elif defined(__linux__) || defined(_LIBCPP_HAS_MUSL_LIBC)
+#define _LIBCPP_ELAST 4095
+#elif defined(__APPLE__)
+// No _LIBCPP_ELAST needed on Apple
+#elif defined(__sun__)
+#define _LIBCPP_ELAST ESTALE
+#elif defined(_LIBCPP_MSVCRT_LIKE)
+#define _LIBCPP_ELAST (_sys_nerr - 1)
+#else
+// Warn here so that the person doing the libcxx port has an easier time:
+#warning ELAST for this platform not yet implemented
+#endif
+
+#endif // _LIBCPP_CONFIG_ELAST