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: NIngo Molnar <mingo@elte.hu>

arch/x86/Kconfig

@@ -18,6 +18,7 @@ config X86_64
 ### Arch settings
 config X86
 	def_bool y
+	select HAVE_UNSTABLE_SCHED_CLOCK
 	select HAVE_IDE
 	select HAVE_OPROFILE
 	select HAVE_KPROBES

init/Kconfig

@@ -316,6 +316,12 @@ config CPUSETS
 
 	  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