LinuxKPI: reduce impact of large MAXCPU

Start scaling arrays dynamically instead of using MAXCPU, resulting in
extra allocations on startup but reducing the overall memory footprint.
For the static single CPU mask we provide two versions to further save
memory depending on a low or high CPU count system. The threshold to
switch is currently at 128 CPUs on 64bit platforms.
More detailed comments on the implementations can be found in the code.

If I am not wrong on a MAXCPU=65536 system the memory footprint should
roughly go down from 512M to 1.5M for the static single CPU mask.

Submitted by: olce (most of this final version)
Sponsored by: The FreeBSD Foundation
PR: 274316
Differential Revision: https://reviews.freebsd.org/D42345