[C++] "ADDRESS_SANITIZER" not defined in fuzzing builds

Fuzzing builds (as run by OSS-Fuzz) enable Address Sanitizer through their own set of options rather than by enabling ARROW_USE_ASAN. However, we need to be informed this situation in the Arrow source code.

One example of where this matters is that eternal thread pools produce spurious leaks at shutdown because of the vector of at-fork handlers; it therefore needs to be worked around on those builds.