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
Add test cases verifying that `CMAKE_APPLE_SILICON_PROCESSOR` set as
either a cache or environment variable causes that to be selected as the
host architecture. Also verify that sources compile using whatever is
selected as the host architecture, even when the explicit setting is not
used.
Issue: #21554
Brad King