sysctl: rename kern_table -> sysctl_subsys_table

Renamed sysctl table from kern_table to sysctl_subsys_table and grouped
the two arch specific ctls to the end of the array.

This is part of a greater effort to move ctl tables into their
respective subsystems which will reduce the merge conflicts in
kernel/sysctl.c.

Signed-off-by: Joel Granados <joel.granados@kernel.org>