Persistent mode is always enabled

After commit `hfuzz-cc: ensure libhfuzz is linked to executables` (904b577c7a381d0eea0539476412787b643c1398) all executables build with `hfuzz-clang` are forced into persistent mode.

Repro:
```
$ cat test.c
int main(void) {
return 0;
}
$ ~/honggfuzz/hfuzz_cc/hfuzz-clang -o test test.c
$ ~/honggfuzz/honggfuzz -i inputs/ -z -s -- ./test
```
Runs in persistent mode and there seems to be no way of disabling it.

Adding a command line option to disable the persistent mode would be one way of fixing it, though I believe the auto-detection is a bit over aggresive :)