After this PR, any macOS tools version bumping in the future will touch fewer files in the repo.
Pointing a Darwin version for the --host system does not matter for the following reasons:
- in terms of the resulted binaries, we should only care about the minimum supported macOS version which is a separated parameter in our build system.
- in terms of the build system itself, the usage of the $(host) variable is self-consistent enough. Btw $(host_os) value already has the version dropped:
This is a backport of core#23585