[Fuchsia] Don't install libc++, libc++abi or libunwind on Darwin

The Clang driver doesn't currently know how to use the libraries
that are shipped as part of the toolchain so there's no reason to
ship them at all.

Differential Revision: https://reviews.llvm.org/D44724

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@328114 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed