arch/x86: Move cpu_relax()

It's not related to spinlocks and the actual implementation
was also guarded by CONFIG(SMP).

With a single call-site in x86-specific code, empty stubs
for other arch are currently not necessary.

Also drop an unused included on a nearby line.

Change-Id: I00439e9c1d10c943ab5e404f5d687d316768fa16
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/43808
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/cpu/x86/tsc/delay_tsc.c

@@ -1,8 +1,7 @@
 /* SPDX-License-Identifier: GPL-2.0-only */
 
+#include <arch/cpu.h>
 #include <cpu/x86/tsc.h>
-#include <pc80/i8254.h>
-#include <smp/spinlock.h>
 #include <delay.h>
 #include <thread.h>