Add env var to toggle large bar support in runtime

Use ROC_ENABLE_LARGE_BAR (0/1) to toggle. The support is
enabled by default.

Change-Id: I6cb93a46594cb6f5e90bf6057738330225efb553


[ROCm/clr commit: d10d691e76]

projects/clr/rocclr/device/rocm/rocdevice.cpp

@@ -896,7 +896,7 @@ hsa_status_t Device::iterateGpuMemoryPoolCallback(hsa_amd_memory_pool_t pool, vo
           if (tmp == HSA_AMD_MEMORY_POOL_ACCESS_NEVER_ALLOWED) {
             dev->info_.largeBar_ = false;
           } else {
-            dev->info_.largeBar_ = true;
+            dev->info_.largeBar_ = ROC_ENABLE_LARGE_BAR;
           }
         }