[libFuzzer] Add support for Fuchsia OS.

Summary:
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

llvm-svn: 320210