Allow mutation of the GDT

[JCapucho]

I've been experiencing with SMP and was trying to reuse the same GDT for all cores since my segments are always the same, but I found a problem, the processor was setting the ACCESS bit in the TSS lower half segment and causing general protection faults when trying to load the TSS segment selector in another processor.

It would be great if there was a way to mutate the GDT in a way that allowed to unset this bit.

[Freax13]

First of all thank you for raising that issue, we were previously not aware of the side effects caused by loading a TSS and the processor changing the GDT entries. The old implementation didn't allow any mutation, not by the processor nor by a user, so given that the processor was mutating it, it was technically unsound.

Following the discussion in #323 (comment), we decided that we will no longer support sharing a GDT in the next breaking release.