x86/asmlinkage: Fix warning in xen asmlinkage change

Current code uses asmlinkage for functions without arguments.
This adds an implicit regparm(0) which creates a warning
when assigning the function to pointers.

Use __visible for the functions without arguments.
This avoids having to add regparm(0) to function pointers.
Since they have no arguments it does not make any difference.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Link: http://lkml.kernel.org/r/1377115662-4865-1-git-send-email-andi@firstfloor.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>