sys/modules/Makefile: Add a section for arm64 + x86 + powerpc64.

This reduces some duplication between the existing arm64 + x86 section
and the powerpc64 section. To make the diff simpler, enable mlx4 on
powerpc64 since it compiles.

Reviewed by: pkubaj, imp, emaste
Differential Revision: https://reviews.freebsd.org/D38973