x86: split loading percpu segments from loading gdt

Impact: split out a function, no functional change

Xen needs to be able to access percpu data from very early on.  For
various reasons, it cannot also load the gdt at that time.   It does,
however, have a pefectly functional gdt at that point, so there's no
pressing need to reload the gdt.

Split the function to load the segment registers off, so Xen can call
it directly.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 1c25eb6..656d02e 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -769,6 +769,7 @@
 
 extern void cpu_set_gdt(int);
 extern void switch_to_new_gdt(int);
+extern void load_percpu_segment(int);
 extern void cpu_init(void);
 
 static inline unsigned long get_debugctlmsr(void)