fuzz: follow modern guidelines and use LIB_FUZZING_ENGINE
This will make our lives easier in an upcoming change, but it's also how we're
supposed to set things up anyway.
Differential Revision: https://phab.mercurial-scm.org/D7561
fuzz: always define LLVMFuzzerInitialize() even if we don't need it
This will make it easier to test our fuzzers outside oss-fuzz.
Differential Revision: https://phab.mercurial-scm.org/D7560