Fix multichar character constant warning

[akien-mga]

Multi-character character constants are implementation defined,
and therefore they are risky to use for use when trying to be compatible
with so many different platforms. Fixes #2540.

I made sure my conversions were correct with:

    print_line(itos(0x31545844)); print_line(itos('1TXD'));
    print_line(itos(0x33545844)); print_line(itos('3TXD'));
    print_line(itos(0x35545844)); print_line(itos('5TXD'));
    print_line(itos(0x31495441)); print_line(itos('1ITA'));
    print_line(itos(0x32495441)); print_line(itos('2ITA'));

Which prints on my system:

    827611204
    827611204
    861165636
    861165636
    894720068
    894720068
    826889281
    826889281
    843666497
    843666497