Since commit b6c60f14b6 (macOS: Default to arm64 architecture on Apple
Silicon hosts, 2020-09-28, v3.19.0-rc1~63^2) we add `-arch arm64` by
default on Apple Silicon hosts if `CMAKE_OSX_ARCHITECTURES` is not set.
This is necessary to prevent the toolchain from selecting its own
default architecture based on that of the build tool (e.g., `x86_64`).
If `CMAKE_<LANG>_COMPILER_TARGET` is set, its `-target` flag tells the
compiler what architecture to use, so do not add `-arch arm64`.
Fixes: #24599
98e319a88f