x86: use stack_start in x86_64

call x86_64's init_rsp stack_start, just as i386 does.
Put a zeroed stack segment for consistency. With this,
we can eliminate one ugly ifdef in smpboot.c.

Signed-off-by: Glauber Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>