[Fuzzer] Use private libc++ even for Fuchsia

On Fuchsia, we use libc++ compiled with ASan for our ASan built
executable which means we cannot use the same libc++ for libFuzzer when
building fuzz targets, instead we'll link a custom internal libc++ into
Fuchsia's build of libFuzzer like we already do on Linux.

Differential Revision: https://reviews.llvm.org/D47835

llvm-svn: 334144