Fixing build by casting IntPtr to nint. (dotnet/coreclr#15530)

Signed-off-by: dotnet-bot <dotnet-bot@microsoft.com>
Signed-off-by: dotnet-bot-corefx-mirror <dotnet-bot@microsoft.com>

Author: dotnet-bot

src/Common/src/CoreLib/Internal/Runtime/CompilerServices/Unsafe.cs

@@ -121,7 +121,7 @@ public static ref T Add<T>(ref T source, IntPtr elementOffset)
             typeof(T).ToString(); // Type token used by the actual method body
             throw new PlatformNotSupportedException();
 #else
-            return ref AddByteOffset(ref source, (IntPtr)(elementOffset * (nint)SizeOf<T>()));
+            return ref AddByteOffset(ref source, (IntPtr)((nint)elementOffset * (nint)SizeOf<T>()));
 #endif
         }