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>

arch/x86/kernel/acpi/sleep.c

@@ -86,7 +86,7 @@ int acpi_save_state_mem(void)
 	saved_magic = 0x12345678;
 #else /* CONFIG_64BIT */
 	header->trampoline_segment = setup_trampoline() >> 4;
-	init_rsp = (unsigned long)temp_stack + 4096;
+	stack_start.sp = temp_stack + 4096;
 	initial_code = (unsigned long)wakeup_long64;
 	saved_magic = 0x123456789abcdef0;
 #endif /* CONFIG_64BIT */