Option --sv-no-strings can cause invalid code to be generated

[lexbailey]

Consider this function, generated from src/sail_sv_backend/generate_primop2.ml:

function automatic sail_unit sail_dec_str_zz5i(logic [127:0] i);
    sail_unit s;
    s.itoa(i);
    return s;
endfunction

s does not have the type string, because of the --sv-no-strings option.
that means that s.itoa is not valid, since that's not defined for the sail_unit type.

Looks to be isolated just to the primops generated in this file.
This and similar functions should probably all just become stubs that return SAIL_UNIT if --sv-no-strings is set?

I can throw together a pull request for this, if that is the problem/solution