Support setting user global pointer value

[xusheng6]

In light of https://github.com/Vector35/binaryninja-api/issues/5617 and https://github.com/Vector35/binaryninja-api/issues/1221, I figured the global pointer value analysis is tricky to do and it is hard to guarantee that our analysis can get the correct value. It would be better if we can offer a way to set it directly