Add flags to run libfuzzer with multiple workers

[killercup]

By default, libfuzzer only runs one thread/worker. We should probably have it default to the number of CPUs and add a flag to set it to a specific number.

[PaulGrandperrin]

same with AFL

[frewsxcv]

AFL's parallel fuzzing story is a little more involved than just turning on a flag, unfortunately :-/

https://github.com/mirrorer/afl/blob/master/docs/parallel_fuzzing.txt