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/arch/arm/include/armv4/arch/smp/spinlock.h

@@ -9,7 +9,6 @@
 #define spin_unlock_wait(lock)	do {} while (0)
 #define spin_lock(lock)		do {} while (0)
 #define spin_unlock(lock)	do {} while (0)
-#define cpu_relax()		do {} while (0)
 
 #include <smp/node.h>
 #define boot_cpu() 1