remove -fstack-clash-protection warnings on FreeBSD/arm64

FreeBSD on arm64 gives warnings like this:

```
DEP gbsinfo.c -o gbsinfo.d
cc: warning: argument unused during compilation: '-fstack-clash-protection' [-Wunused-command-line-argument]
DEP plugout_wav.c -o plugout_wav.d
cc: warning: argument unused during compilation: '-fstack-clash-protection' [-Wunused-command-line-argument]
```

Fix these warnings by not setting -fstack-clash-protection if it would
not get used.

configure

@@ -1126,7 +1126,7 @@ EOF
         append_nodupe CFLAGS "--param=ssp-buffer-size=4"
     fi
     if cc_has_flag "-fstack-clash-protection" \
-       && cc_check "checking if -fstack-clash-protection actually works" "" "-fstack-clash-protection" "yes" "no" <<EOF; then
+       && cc_check "checking if -fstack-clash-protection actually works" "" "-fstack-clash-protection -Wunused-command-line-argument" "yes" "no" <<EOF; then
 int main(int argc, char **argv)
 {
     char buf[65536];