traps: x86_64: make math_state_restore more like i386

- rename variable me -> tsk
 - get thread and tsk like i386
 - expand used_math()
 - copy comment

Signed-off-by: Alexander van Heukelum <heukelum@fastmail.fm>
Signed-off-by: Ingo Molnar <mingo@elte.hu>