Enable kernel dump features in GENERIC for most platforms.

This turns on support for kernel dump encryption and compression, and
netdump. arm and mips platforms are omitted for now, since they are more
constrained and don't benefit as much from these features.

Reviewed by: cem, manu, rgrimes
Tested by: manu (arm64)
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D15465