This switches us to using a LLD based toolchain for macOS builds. Benefits - Less complicated macOS toolchain. - No longer beholden to Apple releasing it's source for cctools, ld64 & libtapi. - No more reliance on third parties to modify those sources for us. i.e apple-libtapi, cctools-port (cctools + ld64).
Backport of core#21778.
Note this needs some adaptation or our cmake build, as well as some update in build.sh to work around a cmake bug with llvm-strip.
The Cross build for OSX will be broken on CI (because it lacks the proper clang version). I paused the build for now and will unbreak in a follow-up.