[CMake] Link static libunwind and libc++abi into libc++ in Fuchsia toolchain

When building libc++ for Fuchsia, we want to distribute shared libc++,
libc++abi and libunwind as separate libraries, but for static versions
we would like to link all of them into libc++ so -lc++ flag has the same
effect whether shared or static library is being used.

Differential Revision: https://reviews.llvm.org/D49628

llvm-svn: 337877