If two threads see _M_index==0 concurrently they will both try to set
it, potentially storing the facet at two different indices in the array.

Either set the _M_index data member using an atomic compare-exchange
operation or while holding a mutex.

Also move the LONG_DOUBLE_COMPAT code into a separate function to remove
the visual noise it creates.

	PR libstdc++/91057
	* src/c++98/locale.cc (locale::id::_M_id()) [__GTHREADS]: Use atomic
	compare-exchange or double-checked lock to ensure only one thread sets
	the _M_index variable.
	[_GLIBCXX_LONG_DOUBLE_COMPAT]: Call find_ldbl_sync_facet to detect
	facets that share another facet's ID.
	[_GLIBCXX_LONG_DOUBLE_COMPAT] (find_ldbl_sync_facet): New function.

From-SVN: r276762