x86: convert to generic helpers for IPI function calls

This converts x86, x86-64, and xen to use the new helpers for
smp_call_function() and friends, and adds support for
smp_call_function_single().

Acked-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index e0edaaa..2f3fbeb 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -168,6 +168,7 @@
 config X86_SMP
 	bool
 	depends on SMP && ((X86_32 && !X86_VOYAGER) || X86_64)
+	select USE_GENERIC_SMP_HELPERS
 	default y
 
 config X86_32_SMP