x86: xen: no need to disable vdso32

Now that the vdso32 code can cope with both syscall and sysenter
missing for 32-bit compat processes, just disable the features without
disabling vdso altogether.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>