[Offload] Make MemoryManager threshold ENV var size_t type. (llvm#124063)

Author: hidekisaito

offload/plugins-nextgen/common/include/MemoryManager.h

@@ -323,7 +323,7 @@ class MemoryManagerTy {
   /// manager explicitly by setting the var to 0. If user doesn't specify
   /// anything, returns <0, true>.
   static std::pair<size_t, bool> getSizeThresholdFromEnv() {
-    static UInt32Envar MemoryManagerThreshold(
+    static UInt64Envar MemoryManagerThreshold(
         "LIBOMPTARGET_MEMORY_MANAGER_THRESHOLD", 0);
 
     size_t Threshold = MemoryManagerThreshold.get();