Xcode: Explicitly specify default native architecture on macOS
CMAKE_OSX_ARCHITECTURES is not specified, we add the Xcode
ONLY_ACTIVE_ARCH = YES with the intention of targeting the
native architecture of the host. However, the default
chosen by "Xcode 12 Universal Apps" includes multiple architectures.
Add an explicit
ARCHS setting with value
to tell Xcode to use the host's native architecture only.
Fixes: #20893 (closed)