libstdc++: Guard mutex and condvar with gthreads macro [PR103638]

A mutex and condition variable is used for timed waits on atomics if
there is no "platform wait" (e.g. futex) supported. But the use of those
types wasn't guarded by the _GLIBCXX_HAS_GTHREADS macro, causing errors
for --disable-threads builds. This fix allows <atomic> to work on
targets with futexes but no gthreads.

libstdc++-v3/ChangeLog:

	PR libstdc++/103638
	* include/bits/atomic_timed_wait.h: Check _GLIBCXX_HAS_GTHREADS
	before using std::mutex and std::__condvar.

libstdc++-v3/include/bits/atomic_timed_wait.h

@@ -137,6 +137,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 // (e.g. __ulock_wait())which is better than pthread_cond_clockwait
 #endif // ! PLATFORM_TIMED_WAIT
 
+#ifdef _GLIBCXX_HAS_GTHREADS
     // Returns true if wait ended before timeout.
     // _Clock must be either steady_clock or system_clock.
     template<typename _Clock, typename _Dur>
@@ -192,6 +193,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	    return false;
 	  }
       }
+#endif // _GLIBCXX_HAS_GTHREADS
 
     struct __timed_waiter_pool : __waiter_pool_base
     {
@@ -239,6 +241,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	if (_M_deadline <= __now)
 	  return false;
 
+	// FIXME: this_thread::sleep_for not available #ifdef _GLIBCXX_NO_SLEEP
+
 	auto __elapsed = __now - _M_t0;
 	if (__elapsed > 128ms)
 	  {