x86: add barriers statement

goal is to have i386 and x86_64 closer, so we
add barriers to match

Signed-off-by: Glauber Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/arch/x86/kernel/smpboot_64.c b/arch/x86/kernel/smpboot_64.c
index d7b59d6..a9cc911 100644
--- a/arch/x86/kernel/smpboot_64.c
+++ b/arch/x86/kernel/smpboot_64.c
@@ -239,6 +239,7 @@
 
 	setup_secondary_clock();
 
+	wmb();
 	cpu_idle();
 }