Apply constexpr on global constant varaibles

When HIP_ENABLE_DEFERRED_LOADING=0, many global variables will be
referenced but they are not initialized in that early time. The patch
will use constexpr to initialze global constant varables in compile
time.

Change-Id: I9d538b7abc6a0ce700ec3332b97fc144db5fc1ef

rocclr/device/gpu/gpuconstbuf.hpp

@@ -30,7 +30,7 @@ namespace gpu {
 class ConstBuffer : public Memory {
  public:
   //! Vector size of the constant buffer
-  static const size_t VectorSize = 16;
+  static constexpr size_t VectorSize = 16;
 
   //! Constructor for the ConstBuffer class
   ConstBuffer(VirtualGPU& gpu,  //!< Virtual GPU device object