cmd/5c: fix arm build

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

LGTM=bradfitz
R=rsc, bradfitz
CC=golang-codereviews
https://golang.org/cl/72220043

src/cmd/5c/txt.c

@@ -31,11 +31,17 @@
 
 #include "gc.h"
 
-LinkArch	*thelinkarch = &linkarm;
 
 int thechar = '5';
 char *thestring = "arm";
 
+LinkArch	*thelinkarch = &linkarm;
+
+void
+linkarchinit(void)
+{
+}
+
 void
 ginit(void)
 {