[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

llvm-svn: 346056