cmd/5c: fix arm build
cmd/5c: fix arm build

cmd/5c/txt.c was missing from CL 72010043.

LGTM=bradfitz
R=rsc, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/72220043

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