[OpenMP] Fixed the link error that cannot find static data member

[tianshilei1992]

Summary

Constant static data member can be defined in the class without another
define after the class in C++17. Although it is C++17, Clang can still handle it
even w/o the flag for C++17. Unluckily, GCC cannot handle that.

[jhuber6]

LGTM.

openmp/libomptarget/plugins/common/MemoryManager/MemoryManager.h

if (const char *Env =
         return std::make_pair(0, false);
       }
     }
 
     return std::make_pair(Threshold, true);
   }
 };
 
+// GCC still cannot handle the static data member like Clang so we still need
+// this part.
+constexpr const size_t MemoryManagerTy::BucketSize[];
+constexpr const int MemoryManagerTy::NumBuckets;
+
 #endif // LLVM_OPENMP_LIBOMPTARGET_PLUGINS_COMMON_MEMORYMANAGER_MEMORYMANAGER_H