Since commit b6c60f14b6 (macOS: Default to arm64 architecture on Apple
Silicon hosts, 2020-09-28, v3.19.0-rc1~63^2) we use `sysctl` to detect
that we are running on Apple Silicon in a way that pierces Rosetta.
This always sets `CMAKE_HOST_SYSTEM_PROCESSOR` to be `arm64` on such
hosts. However, macOS offers strong support for running processes under
an emulated `x86_64` architecture.
Teach CMake to select either `arm64` or `x86_64` as the host
architecture on Apple Silicon based on the architecture of its own
process. When CMake is built as a universal binary, macOS will select
whichever slice (architecture) is appropriate under the user's shell,
and `CMAKE_HOST_SYSTEM_PROCESSOR` will match.
Also offer a `CMAKE_APPLE_SILICON_PROCESSOR` variable and environment
variable to provide users with explicit control over the host
architecture selection regardless of CMake's own architecture.
Finally, if `CMAKE_OSX_ARCHITECTURES` is not set, pass explicit flags to
the toolchain to use selected host architecture instead of letting the
toolchain pick.
Fixes: #21554
Currently the document says that `CMAKE_HOST_SYSTEM_PROCESSOR` is the
output of `uname -p` if a system supports `uname`. Update the
documented behavior to match `CMakeDetermineSystem.cmake` code.
Fixes: #20004
Improve formatting, primarily by:
* Adding links to relevant commands, properties, generators, and so on.
* Converting code, symbols, paths, and so on to fixed-width fonts.
* Hard wrapping lines to 80 characters or less.
Brad King