godex: improved printing of numeric constants
godex: improved printing of numeric constants

TODO: Add testing code (another CL).

LGTM=adonovan
R=golang-codereviews, adonovan
CC=golang-codereviews
https://codereview.appspot.com/88090044

t-yuki/go.tools.old,t-yuki/go.tools.old,t-yuki/go.tools.old