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/optional.cc

@@ -0,0 +1,42 @@
+// clang-format off
+//===------------------------ optional.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/optional"
+
+namespace std
+{
+
+bad_optional_access::~bad_optional_access() _NOEXCEPT = default;
+
+const char* bad_optional_access::what() const _NOEXCEPT {
+  return "bad_optional_access";
+  }
+
+} // std
+
+
+#include "third_party/libcxx/experimental/__config"
+
+//  Preserve std::experimental::bad_optional_access for ABI compatibility
+//  Even though it no longer exists in a header file
+_LIBCPP_BEGIN_NAMESPACE_EXPERIMENTAL
+
+class _LIBCPP_EXCEPTION_ABI _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS bad_optional_access
+    : public std::logic_error
+{
+public:
+	bad_optional_access() : std::logic_error("Bad optional Access") {}
+
+//	Get the key function ~bad_optional_access() into the dylib
+    virtual ~bad_optional_access() _NOEXCEPT;
+};
+
+bad_optional_access::~bad_optional_access() _NOEXCEPT = default;
+
+_LIBCPP_END_NAMESPACE_EXPERIMENTAL