no Perl_is_uni_* API with -DNO_MATHOMS

[rurban]

nm "`cperl5.28.1-nt -V::archlib:`/CORE/`cperl5.28.1-nt -V::libperl:`" | grep Perl_is_uni_ is missing, because of -DNO_MATHOMS. compare to a perl without -DNO_MATHOMS, e.g. Perl_is_uni_upper

This is needed for Perl-Tk, ie Perl-Tk fails to build with -DNO_MATHOMS. Most other extensions build fine with -DNO_MATHOMS.