sysctl: mark more nodes as MPSAFE

vm.kvm_size and vm.kvm_free are read only and marked as MPSAFE on i386
already. Mark them as that on amd64 and arm64 too to avoid locking Giant.

Reviewed by: kib (mentor)
Approved by: kib (mentor)
Differential Revision: https://reviews.freebsd.org/D23039