Use lto.h from clang+llvm not libtapi. The later is older,
and comes bundled with the libtapi repo.
Copy libLTO.so when building with FORCE_USE_SYSTEM_CLANG.
If we're cross-compiling for darwin on aarch64 hardware, we need need to
use a Clang that will run on that hardware.
Only tested in a Linux Docker container (aarch64-unknown-linux-gnu),
running on an Apple M1 mac-mini (aarch64-apple-darwin20.5.0).
