[CMake] Use libc++ and compiler-rt as default libraries in Fuchsia toolchain
[CMake] Use libc++ and compiler-rt as default libraries in Fuchsia toolchain

Fuchsia already defaults to libc++ and compiler-rt, but we want to use
these as default runtimes even on the host platform.

Differential Revision: https://reviews.llvm.org/D39930

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@327860 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang