[CMake][Fuchsia] Set -fuse-ld=lld explicitly for Linux runtimes
[CMake][Fuchsia] Set -fuse-ld=lld explicitly for Linux runtimes

We set lld as the default linker on non-Darwin platforms, but we still
need to set -fuse-ld=lld explicitly in to support cross-compiling Linux
runtimes on Darwin.

Differential Revision: https://reviews.llvm.org/D54026

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@346056 91177308-0d34-0410-b5e6-96231b3b80d8

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