Install the linux(4) man page also for aarch64

[trasz]

Summary

Previously it was only installed on i386 and amd64.

[emaste]

linux.4 also needs some updating, e.g.

To compile support for this ABI into an i386 kernel place the following
line in your kernel configuration file:

      options COMPAT_LINUX

for an amd64 kernel use:

      options COMPAT_LINUX32

Alternatively, to load the ABI as a module at boot time, place the
following line in loader.conf(5):

      linux_load="YES"

[trasz]

(Tinderboxed.)

[gbe]

LGTM