lib9: restore printing of signed integers
lib9: restore printing of signed integers

A casualty of https://codereview.appspot.com/10195044.

If x is an 32-bit int and u is a 64-bit ulong,
        u = (uint)x // converts to uint before extension, so zero fills
        u = (ulong)x // sign-extends

TBR=iant, r
CC=golang-dev
https://codereview.appspot.com/10814043

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang