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/chrono.cc

@@ -0,0 +1,68 @@
+// clang-format off
+//===------------------------- chrono.cpp ---------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "third_party/libcxx/chrono"
+#include "third_party/libcxx/cerrno"        // errn"
+#include "libc/sysv/consts/clock.h"
+#include "libc/time/time.h"
+#include "libc/sysv/consts/clock.h"
+#include "third_party/libcxx/system_error"  // __throw_system_erro"
+
+#define _LIBCPP_USE_CLOCK_GETTIME
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+namespace chrono
+{
+
+// system_clock
+
+const bool system_clock::is_steady;
+
+system_clock::time_point
+system_clock::now() _NOEXCEPT
+{
+  struct timespec tp;
+  if (0 != clock_gettime(CLOCK_REALTIME, &tp))
+    __throw_system_error(errno, "clock_gettime(CLOCK_REALTIME) failed");
+  return time_point(seconds(tp.tv_sec) + microseconds(tp.tv_nsec / 1000));
+}
+
+time_t
+system_clock::to_time_t(const time_point& t) _NOEXCEPT
+{
+    return time_t(duration_cast<seconds>(t.time_since_epoch()).count());
+}
+
+system_clock::time_point
+system_clock::from_time_t(time_t t) _NOEXCEPT
+{
+    return system_clock::time_point(seconds(t));
+}
+
+// steady_clock
+//
+// Warning:  If this is not truly steady, then it is non-conforming.  It is
+//  better for it to not exist and have the rest of libc++ use system_clock
+//  instead.
+
+const bool steady_clock::is_steady;
+
+steady_clock::time_point
+steady_clock::now() _NOEXCEPT
+{
+    struct timespec tp;
+    if (0 != clock_gettime(CLOCK_MONOTONIC, &tp))
+        __throw_system_error(errno, "clock_gettime(CLOCK_MONOTONIC) failed");
+    return time_point(seconds(tp.tv_sec) + nanoseconds(tp.tv_nsec));
+}
+
+}  // namespace chrono
+
+_LIBCPP_END_NAMESPACE_STD