x86: change init_gdt to update the gdt via write_gdt, rather than a direct write.

By writing directly, a memory access violation can occur whilst
hotplugging a CPU if the entry was previously marked read-only.

Signed-off-by: Alex Nixon <alex.nixon@citrix.com>
Cc: Jeremy Fitzhardinge <Jeremy.Fitzhardinge@citrix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>