Bug 40605: Reworked the macOS toolchain creation.
We were compiling another Clang with the macOS toolchain, but we did not really need to, so I have removed that step.
However, compiling compiler-rt was failing in this way, so I have disabled a pair of components we should not need. Probably, only the built-ins are actually needed.
Finally, we now compile libc++ in the way LLVM currently suggests.
Fixes: #40605 (closed).
Edited by Pier Angelo Vendrame