x86: move __get_user and __put_user into uaccess.h.

We also carry the unaligned version with us. Only x86_64 uses
it, but there's no problem in defining it.

Signed-off-by: Glauber Costa <gcosta@redhat.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>