cmd/gc: avoid %L in error message

Fixes #7783

LGTM=minux.ma
R=rsc, minux.ma
CC=golang-codereviews
https://golang.org/cl/89290043

src/cmd/gc/lex.c

@@ -214,7 +214,7 @@ main(int argc, char *argv[])
 	// but not other values.	
 	p = getgoarch();
 	if(strncmp(p, thestring, strlen(thestring)) != 0)
-		fatal("cannot use %cg with GOARCH=%s", thechar, p);
+		sysfatal("cannot use %cg with GOARCH=%s", thechar, p);
 	goarch = p;
 
 	linkarchinit();