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 <jdike@addtoit.com>
Cc: Richard Weinberger <richard@nod.at>
Link: http://lkml.kernel.org/r/tip-45db1c6176c8171d9ae6fa6d82e07d115a5950ca@git.kernel.org
Signed-off-by: H. Peter Anvin <hpa@zytor.com>