[Fuzzer] When building for Fuchsia, add an explicit libc++ dependency

libFuzzer dependes on C++ library, when building for Fuchsia, we
need to ensure that libFuzzer is only being built after libc++
has been built, so we add an explicity dependency on it.

Differential Revision: https://reviews.llvm.org/D44340

llvm-svn: 327196