x86: re-enable NX if disabled

x86: re-enable NX if disabled

I noticed Linux 4.4 doing this universally now, and I think it's a good
idea to override such anti-security BIOS settings (we certainly have no
compatibility problem due to NX being enabled).

Secondary changes:
- no need to check supported extended CPUID level for leaves 80000000
  and 80000001 (required on x86-64)
- no need to update c->cpuid_level in early_init_intel() (done anyway
  in generic_identify())
- alignment of trampoline data items

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>