Unfortunately clang 10 does not understand "-mmacosx-version-min=11.0",
as it expects to see only 10.x.
Bump minimally to 11.1 to fix that problem. This will likely be our last
binary toolchain bump, as it will soon be replaced with usage of upstream
vanilla llvm.
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).
