lib9: restore argv0 initialization code.

`GOARCH=arm go tool 6c` used to give "<prog>: cannot use 6c with GOARCH=arm"

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews
https://golang.org/cl/89330043

src/lib9/main.c

@@ -52,8 +52,8 @@ main(int argc, char **argv)
 	// don't display the crash dialog
 	DWORD mode = SetErrorMode(SEM_NOGPFAULTERRORBOX);
 	SetErrorMode(mode | SEM_FAILCRITICALERRORS | SEM_NOGPFAULTERRORBOX | SEM_NOOPENFILEERRORBOX);
-	argv0 = argv[0];
 #endif
+	argv0 = argv[0];
 	p9main(argc, argv);
 	exits("main");
 	return 99;