[CMake] Use lld and llvm-objcopy for first stage compiler in Fuchsia

When cross-compiling the second stage to a different target, we need to
make sure that the first-stage compiler can produce binaries for that
target. Using lld and llvm-objcopy as the default linker and objcopy
tool eliminates some of the dependencies on the host toolchain.

Differential Revision: https://reviews.llvm.org/D54655

llvm-svn: 347108