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/platform/agent.cpp

@@ -276,7 +276,7 @@ static int32_t CL_API_CALL SetICDDispatchTable(vdi_agent* agent, const cl_icd_di
 
 }  // namespace agent
 
-vdi_agent Agent::entryPoints_ = {agent::GetVersionNumber,
+const vdi_agent Agent::entryPoints_ = {agent::GetVersionNumber,
                                 agent::GetPlatform,
                                 agent::GetTime,
                                 agent::SetCallbacks,