tests: move the fuzzer upstream

It should make it easier to start using CFLite or something like that
to fuzz libbpf without getting pointless CVEs :-) More importantly,
now it's possible to build the fuzzer by just cloning the repository,
installing clang and running `./scripts/build-fuzzers.h`:
```
git clone https://github.com/libbpf/libbpf
./scripts/build-fuzzers.h
unzip -d CORPUS fuzz/bpf-object-fuzzer_seed_corpus.zip
./out/bpf-object-fuzzer CORPUS
```

It should make it easier (for me at least) to report some
elfutils bugs because they are much easier to reproduce manually
now.

fuzz/bpf-object-fuzzer.c

@@ -0,0 +1,23 @@
+#include "libbpf.h"
+
+static int libbpf_print_fn(enum libbpf_print_level level, const char *format, va_list args)
+{
+	return 0;
+}
+
+int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
+	struct bpf_object *obj = NULL;
+	DECLARE_LIBBPF_OPTS(bpf_object_open_opts, opts);
+	int err;
+
+	libbpf_set_print(libbpf_print_fn);
+
+	opts.object_name = "fuzz-object";
+	obj = bpf_object__open_mem(data, size, &opts);
+	err = libbpf_get_error(obj);
+	if (err)
+		return 0;
+
+	bpf_object__close(obj);
+	return 0;
+}