[libc++] Move once_flag outside of <mutex>

This allows including once_flag directly from <__locale> instead of
depending on all of <mutex>, which requires threading. In turn, this
makes it easier to support locales on platforms without threading.

Drive-by change: clang-format once_flag.h and use _LIBCPP_HIDE_FROM_ABI

Differential Revision: https://reviews.llvm.org/D155487