Fix leading comma it expression only has "others" part

Stop syntax errors caused by things like this:
  (, others => '1')

tgt-vhdl/vhdl_syntax.cc

@@ -768,7 +768,7 @@ void vhdl_bit_spec_expr::emit(std::ostream &of, int level) const
    }
 
    if (others_) {
-      of << ", others => ";
+      of << (bits_.empty() ? "" : ", ") << "others => ";
       others_->emit(of, level);
    }