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/rocm/rocdevice.cpp

@@ -68,8 +68,6 @@ bool roc::Device::isHsaInitialized_ = false;
 std::vector<hsa_agent_t> roc::Device::gpu_agents_;
 std::vector<AgentInfo> roc::Device::cpu_agents_;
 
-const bool roc::Device::offlineDevice_ = false;
-const bool roc::NullDevice::offlineDevice_ = true;
 address Device::mg_sync_ = nullptr;
 
 static HsaDeviceId getHsaDeviceId(hsa_agent_t device, uint32_t& pci_id) {