[libFuzzer] Add support for Fuchsia OS.
This patch adds the initial support for Fuchsia.
- LIBFUZZER_FUCHSIA is added as an OS type in FuzzerDefs.h
- Fuchsia is, by design, not POSIX compliant. However, it does use ELF and supports common POSIX I/O functions. Thus, FuzzerExtFunctions.h and FuzzerIO.h are implemented by extending the header guards in FuzzerExtFunctionsWeak.cpp and FuzzerIOPosix.cpp to include LIBFUZZER_FUCHSIA.
- The platform-specific portions of FuzzerUtil.h are implemented by FuzzerUtilFuchsia.cpp, which makes use of exception ports, syscalls, and the launchpad library.
- The experimental equivalence server is not currently supported, so FuzzerShmem.h is implemented by stub methods in FuzzerShmemFuchsia.cpp. Any future implementation will likely involve VMOs.
Tested with ASAN/SanCov on Fuchsia/x86-64 with the canonical toy fuzzer.
Patch By: aarongreen
Reviewers: kcc, morehouse, flowerhack, phosek
Reviewed By: kcc, phosek, Eugene.Zelenko
Subscribers: srhines, mgorny, Eugene.Zelenko
Differential Revision: https://reviews.llvm.org/D40974