Install the linux(4) man page also for aarch64

Previously it was only installed on i386 and amd64.

Reviewed By: emaste, gbe (manpages)
Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D30546

(cherry picked from commit dd869341b1e0b58eb50fd2700b3191094d97ef5b)