sched, x86: add HAVE_UNSTABLE_SCHED_CLOCK

add the HAVE_UNSTABLE_SCHED_CLOCK, for architectures to select.

the next change utilizes it.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 845ea2b2..bbcafaa 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -18,6 +18,7 @@
 ### Arch settings
 config X86
 	def_bool y
+	select HAVE_UNSTABLE_SCHED_CLOCK
 	select HAVE_IDE
 	select HAVE_OPROFILE
 	select HAVE_KPROBES
diff --git a/init/Kconfig b/init/Kconfig
index f0e62e5..fa42e6b 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -316,6 +316,12 @@
 
 	  Say N if unsure.
 
+#
+# Architectures with an unreliable sched_clock() should select this:
+#
+config HAVE_UNSTABLE_SCHED_CLOCK
+	bool
+
 config GROUP_SCHED
 	bool "Group CPU scheduler"
 	default y