[CMake][Fuchsia] Use libc++ ABI v2 on Darwin as well
[CMake][Fuchsia] Use libc++ ABI v2 on Darwin as well

Since we share headers between host and target builds, we need to use
the same version for both.

Differential Revision: https://reviews.llvm.org/D62712

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@362181 91177308-0d34-0410-b5e6-96231b3b80d8

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