cpu_idle_job: relax a bit

This *dramatically* improves kernel boot time with GCOV builds

from ~3minutes between loading kernel and switching the HILE
bit down to around 10 seconds.

Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>

core/cpu.c

@@ -450,6 +450,7 @@ void cpu_idle_job(void)
 		while (!cpu_check_jobs(cpu)) {
 			if (pm_enabled)
 				break;
+			cpu_relax();
 			barrier();
 		}
 		smt_medium();