When `CMAKE_OSX_ARCHITECTURES` is not specified, we add the Xcode
setting `ONLY_ACTIVE_ARCH = YES` with the intention of targeting the
native architecture of the host.  However, the default `ARCHS` value
chosen by "Xcode 12 Universal Apps" includes multiple architectures.
Add an explicit `ARCHS` setting with value `$(NATIVE_ARCH_ACTUAL)`
to tell Xcode to use the host's native architecture only.

Fixes: #20893