Set RSP0 on context switch

Previously we only set it on thread init which caused interrupts to
stomp on the stack of the most recently started thread, potentially
overwriting their cr3 and other variables and causing a crash.

zion/scheduler/scheduler.cpp

@@ -1,5 +1,6 @@
 #include "scheduler/scheduler.h"
 
+#include "common/gdt.h"
 #include "debug/debug.h"
 #include "lib/linked_list.h"
 #include "scheduler/process_manager.h"
@@ -26,6 +27,7 @@ void Scheduler::SwapToCurrent(Thread& prev) {
   }
   current_thread_->SetState(Thread::RUNNING);
 
+  SetRsp0(current_thread_->Rsp0Start());
   context_switch(prev.Rsp0Ptr(), current_thread_->Rsp0Ptr());
 
   asm volatile("sti");