arch/{arm,arm64,ppc64,riscv}: Add noop cpu_relax

The cpu_relax method is defined for x86. This CL adds a no-op method so
that it can be used in common code.

BUG=b:179699789
TEST=none

Signed-off-by: Raul E Rangel <rrangel@chromium.org>
Change-Id: Ifcb4546ceb2894eeb37589d0282b7e076d7a4747
Reviewed-on: https://review.coreboot.org/c/coreboot/+/59546
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

src/arch/arm/include/armv4/arch/cpu.h

@@ -3,6 +3,8 @@
 #ifndef __ARCH_CPU_H__
 #define __ARCH_CPU_H__
 
+static inline void cpu_relax(void) { }
+
 #define asmlinkage
 
 #endif /* __ARCH_CPU_H__ */

src/arch/arm/include/armv7/arch/cpu.h

@@ -6,6 +6,8 @@
 #include <stdint.h>
 #include <device/device.h>
 
+static inline void cpu_relax(void) { }
+
 #define asmlinkage
 
 struct cpu_driver {

src/arch/arm64/include/armv8/arch/cpu.h

@@ -3,6 +3,9 @@
 #ifndef __ARCH_CPU_H__
 #define __ARCH_CPU_H__
 
+/* TODO: Implement using SEV/WFE if this is ever actually used. */
+static inline void cpu_relax(void) { }
+
 #define asmlinkage
 
 struct cpu_driver { };

src/arch/ppc64/include/arch/cpu.h

@@ -5,6 +5,8 @@
 
 #include <device/device.h>
 
+static inline void cpu_relax(void) { }
+
 #define asmlinkage
 
 struct cpu_driver {

src/arch/riscv/include/arch/cpu.h

@@ -6,6 +6,8 @@
 #include <arch/encoding.h>
 #include <device/device.h>
 
+static inline void cpu_relax(void) { }
+
 #define asmlinkage
 
 struct cpu_driver {