hw_x86_64: Use PAUSE instruction in wait_for_interrupt

Add spin loop hint by means of the PAUSE instruction since
wait_for_interrupt is called in a busy loop. This should improve processor
performance and reduce power consumption.

Note: HLT cannot be used since it is a privileged instruction and the idle
      thread is executed in userspace.

repos/base-hw/src/core/include/spec/x86/cpu_support.h

@@ -404,7 +404,7 @@ class Genode::Cpu
 		/**
 		 * Wait for the next interrupt as cheap as possible
 		 */
-		static void wait_for_interrupt() { }
+		static void wait_for_interrupt() { asm volatile ("pause"); }
 
 		/**
 		 * Return wether to retry an undefined user instruction after this call