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

d0f/go-zh,rdp/rogerpack2005-golang,bryanxu/go-zh,webfd/go-zh,webfd/go-zh,webfd/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,bryanxu/go-zh,bryanxu/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,webfd/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,webfd/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,webfd/go-zh,bryanxu/go-zh,d0f/go-zh,bryanxu/go-zh,d0f/go-zh,d0f/go-zh,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,d0f/go-zh,rdp/rogerpack2005-golang,d0f/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc