x86, um: Mark system call tables readonly

x86, um: Mark system call tables readonly

Mark the system call tables readonly, as they already are on native,
and the 32-bit UM version was in the previous assembly version.  The
32-bit version lost it due to copy and paste from the 64-bit version,
which was missing the const.

Cc: Jeff Dike <[email protected]>
Cc: Richard Weinberger <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: H. Peter Anvin <[email protected]>