arm: Move spin() stub to the raspberry_pi cpu.cpp

This unbreaks the U-Boot build, since we already have spin()
in cpu.cpp there, for future implementation using the U-Boot API.

src/system/boot/arch/arm/kernel_stubs.cpp

@@ -33,12 +33,6 @@ kgetc()
 }
 
 
-extern "C" void
-spin(bigtime_t time)
-{
-}
-
-
 extern "C" status_t
 _mutex_lock(mutex*, bool)
 {

src/system/boot/platform/raspberrypi_arm/cpu.cpp

@@ -6,6 +6,14 @@
 
 #include "arch_cpu.h"
 
+extern "C" void
+spin(bigtime_t microseconds)
+{
+	// fallback to arch-specific code
+	arch_spin(microseconds);
+}
+
+
 extern "C" void
 cpu_init()
 {