x86/xen: Initialize cr4 shadow for 64-bit PV(H) guests

Commit 1e02ce4c ("x86: Store a per-cpu shadow copy of CR4")
introduced CR4 shadows.

These shadows are initialized in early boot code. The commit missed
initialization for 64-bit PV(H) guests that this patch adds.
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

arch/x86/xen/enlighten.c

@@ -1758,6 +1758,7 @@ asmlinkage __visible void __init xen_start_kernel(void)
 #ifdef CONFIG_X86_32
 	i386_start_kernel();
 #else
+	cr4_init_shadow(); /* 32b kernel does this in i386_start_kernel() */
 	x86_64_start_reservations((char *)__pa_symbol(&boot_params));
 #endif
 }