add weak and strong to Utils.v_keywords (#1983) (#1987)

Verilator 4.034 was complaining about wires being named weak and strong
because those are SV 2009 keywords.  Added them to the Utils.v_keywords list

(cherry picked from commit 3be73de2

)

Co-authored-by: Tim Snyder <timothy.snyder@sifive.com>