Prevent implicit -std=* passing from NXCFLAGS to NXCXXFLAGS.
C and c++ standards are not cross compatible, this is a major issue with clang
import into the base that explictly requires at least the -std=c++11 standard.
It gets complicated because even tablegen utilities used as buildtools depend
explictly on c++11. This implies that we no longer can avoid libstdc++ usage
in hostlib/hostprog infrastructure.
While there, explcitly filter out -flto options from host native tools FLAGS
and add a missing rule to allow generating host native c++ utilities for the
use in buildworld.