Don't symlink fusefs.ko to fuse.ko on PPC

Some PPC systems (PowerNV) use msdosfs for /boot, which can't handle either
symlinks or hardlinks. So on PPC, copy the module instead. This change fixes
installkernel on such systems after r345350.

Reported by: Brandon Bergren <git_bdragon.rtk0.net>
Reviewed by: jhibbits, rgrimes
MFC after: 2 weeks
MFC-With: 345350, 346441
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D19993