QThread/Unix: intercept exceptions only with Glibc

See 5677b70 for details why we need
such specific exception handling for Glibc. In short, on Glibc,
pthread_cancel and pthread_exit are implemented by throwing a special
kind of exception that can be caught, but must always be rethrown. That
exception is then used to activate the cancellation clean-up handlers.

But for libc++ on Apple platforms we can simply wrap it as noexcept
to guarantee that std::terminate() will be called in case of any
unhandled exception.

Task-number: QTBUG-141803
Pick-to: 6.10 6.8
Change-Id: Iaa88d3a8091425206ee2735e835ae74fd087e9e0
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

Author: VladimirBelyavsky

src/corelib/thread/qthread_unix.cpp

@@ -358,26 +358,28 @@ static void setCurrentThreadName(const char *name)
 #endif
 
 namespace {
+#if defined(__GLIBCXX__) && !defined(QT_NO_EXCEPTIONS)
 template <typename T>
 void terminate_on_exception(T &&t)
 {
-#ifndef QT_NO_EXCEPTIONS
     try {
-#endif
         std::forward<T>(t)();
-#ifndef QT_NO_EXCEPTIONS
-#ifdef __GLIBCXX__
-    // POSIX thread cancellation under glibc is implemented by throwing an exception
-    // of this type. Do what libstdc++ is doing and handle it specially in order not to
-    // abort the application if user's code calls a cancellation function.
     } catch (abi::__forced_unwind &) {
+        // POSIX thread cancellation under glibc is implemented by throwing an exception
+        // of this type. Do what libstdc++ is doing and handle it specially in order not to
+        // abort the application if user's code calls a cancellation function.
         throw;
-#endif // __GLIBCXX__
     } catch (...) {
         std::terminate();
     }
-#endif // QT_NO_EXCEPTIONS
 }
+#else
+template <typename T>
+void terminate_on_exception(T &&t) noexcept
+{
+    std::forward<T>(t)();
+}
+#endif // defined(__GLIBCXX__) && !defined(QT_NO_EXCEPTIONS)
 } // unnamed namespace
 
 void *QThreadPrivate::start(void *arg)