x86/xen: init %gs very early to avoid page faults with stack protector

x86/xen: init %gs very early to avoid page faults with stack protector

When running as Xen pv guest %gs is initialized some time after
C code is started. Depending on stack protector usage this might be
too late, resulting in page faults.

So setup %gs and MSR_GS_BASE in assembly code already.

Cc: [email protected]
Signed-off-by: Juergen Gross <[email protected]>
Reviewed-by: Boris Ostrovsky <[email protected]>
Tested-by: Chris Patterson <[email protected]>
Signed-off-by: Juergen Gross <[email protected]>