x86: Remove unnecessary compile flag tweaks for vsyscall code

As of commit 98d0ac38ca7b1b7a552c9a2359174ff84decb600
Author: Andy Lutomirski <luto@mit.edu>
Date:   Thu Jul 14 06:47:22 2011 -0400

    x86-64: Move vread_tsc and vread_hpet into the vDSO

user code no longer directly calls into code in arch/x86/kernel/, so
we don't need compile flag hacks to make it safe.  All vdso code is
in the vdso directory now.

Signed-off-by: Andy Lutomirski <luto@mit.edu>
Link: http://lkml.kernel.org/r/835cd05a4c7740544d09723d6ba48f4406f9826c.1312988155.git.luto@mit.edu
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>