contrib/mandoc: add -ieee754-2008

This is needed to simplify the msun manpages {sin,cos,tan}pi.3

Approved by: markj (mentor)
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D53784

(cherry picked from commit db3884b03989b095d746087afab8e5e649c3699a)