arch: Move the gdt module to the hypervisor crate

We will need the GDT API for the hypervisor's x86 instruction
emulator implementation, it's better if the arch crate depends on the
hypervisor one rather than the other way around.

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

arch/src/x86_64/mod.rs

@@ -7,7 +7,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE-BSD-3-Clause file.
 use std::sync::Arc;
-mod gdt;
 pub mod interrupts;
 pub mod layout;
 mod mptable;