x86/asm: Unify segment selector defines

Those are identical on 32- and 64-bit, unify them. No functional
change.

Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1418127959-29902-1-git-send-email-bp@alien8.de


Signed-off-by: Ingo Molnar <mingo@kernel.org>