Add FIB_ALGO to GENERIC on amd64/arm64.

Option FIB_ALGO gates new modular fib lookup functionality,
enabling more performant routing table lookups and improving
control plane convergence under the load.

Detailed feature description is available in D27401.

Reviewed By: olivier, gnn
Differential Revision: https://reviews.freebsd.org/D28434

(cherry picked from commit 6993187a8c30e83a408aad631a8d8629d8273c9d)