Add libFuzzer shared object build output

This change adds a CMake rule to produce shared object versions of
libFuzzer (no-main). Like the static library versions, these shared
libraries have a copy of libc++ statically linked in. For i386 we don't
link with libc++ since i386 does not support mixing position-
independent and non-position-independent code in the same library.

Patch By: IanPudney

Reviewed By: morehouse

Differential Revision: https://reviews.llvm.org/D84947