Dynamically determine the operating system on win7

[ZhuWenhao-88]

Although #509 has been resolved, there is still a problem. For some reasons, We will build .julia in advance on win10 and then use it on win 7/win 10/win 11, which will cause the jldopen function of JLD2 on win 7 to be unavailable.Therefore, I need to dynamically determine the operating system to initialize DEFAULT_IOTYPE when JLD2 is initialized.