[kernel] Set process stack pointer correctly

The rsp returned by initialize_main_user_stack() needs to be put into
the cpu data area, not just put into the stack (the stack only fills in
rbp).

src/kernel/task.s

@@ -63,6 +63,7 @@ initialize_main_thread:
 
 	; user rsp is now in rax, put it in the right place for sysret
 	mov [rsp + 0x30], rax
+	mov [gs:CPU_DATA.rsp3], rax
 
 	; the entrypoint should already be on the stack
 	jmp kernel_to_user_trampoline