libstdc++: Add missing return and use reserved name

libstdc++: Add missing return and use reserved name

The once_flag::_M_activate() function is only ever called immediately
after a call to once_flag::_M_passive(), and so in the non-gthreads case
it is impossible for _M_passive() to be true in the body of
_M_activate(). Add a check for it anyway, to avoid warnings about
missing return.

Also replace a non-reserved name with a reserved one.

libstdc++-v3/ChangeLog:

	* include/std/mutex (once_flag::_M_activate()): Add explicit
	return statement for passive case.
	(once_flag::_M_finish(bool)): Use reserved name for parameter.