Disable FORTIFY for fuzzer builds.

Disables FORTIFY for fuzz target builds. We generally build with a
sanitizer and want the sanitizer to handle the bugs and provide us with
a nice stack trace.

Bug: 140187921
Test: m example_fuzzer && $ANDROID_HOST_OUT/fuzz/example_fuzzer (note
that you get a proper ASan report instead of a fortify crash)

Change-Id: I15a117b26c401a03785fb44c0f796588f366bc55