Manually clearing the global device vector

The vector of devices is a global object, which destruction order is
undefined. In some platforms, the OpenCL library has been unloaded
before this destructor is hit, which causes a segmentation fault after
the program ends.  By clearing the global vector of devices on
destruction of the OpenCL and SYCL Stream benchmarks we avoid the
problem.

OCLStream.cpp

@@ -186,6 +186,8 @@ OCLStream<T>::~OCLStream()
   delete mul_kernel;
   delete add_kernel;
   delete triad_kernel;
+
+  devices.clear();
 }
 
 template <class T>

SYCLStream.cpp

@@ -94,6 +94,7 @@ SYCLStream<T>::~SYCLStream()
 
   delete p;
   delete queue;
+  devices.clear();
 }
 
 template <class T>
@@ -322,7 +323,6 @@ std::string getDeviceDriver(const int device)
   return driver;
 }
 
-
 // TODO: Fix kernel names to allow multiple template specializations
 template class SYCLStream<float>;
 template class SYCLStream<double>;