libstdc++: Refactor std::call_once internals

libstdc++: Refactor std::call_once internals

This separates the definition of std::__call_proxy into two funcions,
one for TLS and one for non-TLS, to make them easier to read. It also
replaces the __get_once_functor_lock_ptr() internal helper with a new
set_lock_ptr(unique_lock<mutex>*) function so that __once_proxy doesn't
need to call it twice.

libstdc++-v3/ChangeLog:

	* src/c++11/mutex.cc [_GLIBCXX_HAVE_TLS] (__once_proxy): Define
	separately for TLS targets.
	[!_GLIBCXX_HAVE_TLS] (__get_once_functor_lock_ptr): Replace with ...
	(set_lock_ptr): ... this. Set new value and return previous
	value.
	[!_GLIBCXX_HAVE_TLS] (__set_once_functor_lock_ptr): Adjust to
	use set_lock_ptr.
	[!_GLIBCXX_HAVE_TLS] (__once_proxy): Likewise.