Xcode: Explicitly specify default native architecture on macOS

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
29 jobs for !5023 with xcode-native-arch in 7 minutes and 16 seconds (queued for 3 seconds)
detached
Status Name Job ID Coverage
  Build
manual build:centos6-release #5088698
build docker linux manual
passed build:debian10-iwyu #5088695
build docker linux

00:01:40

manual build:fedora31-ninja #5088699
build docker linux linux-3.17 manual
passed build:fedora31-sphinx #5088697
build docker linux linux-3.17

00:04:44

passed build:fedora31-tidy #5088696
build docker linux linux-3.17

00:07:16

manual build:macos-makefiles #5088701
cmake macos shell xcode-11.5 nonconcurrent manual
manual build:macos-ninja #5088700
cmake macos shell xcode-11.5 nonconcurrent manual
manual build:windows-vs2019-x64-ninja #5088702
cmake windows shell vs2019 msvc-19.25 nonconcurrent manual
 
  Test
created test:fedora31-ninja #5088703
build docker linux linux-3.17
created test:macos-makefiles #5088705
cmake macos shell xcode-11.5 nonconcurrent
created test:macos-ninja #5088704
cmake macos shell xcode-11.5 nonconcurrent
created test:windows-vs2019-x64-ninja #5088706
cmake windows shell vs2019 msvc-19.25 nonconcurrent
 
  Test Ext
created test:cuda10.2-nvidia #5088708
cuda-rt docker linux
created test:fedora31-makefiles #5088707
build docker linux linux-3.17
created test:fedora31-ninja-multi #5088709
build docker linux linux-3.17
created test:macos-xcode #5088710
cmake macos shell xcode-11.5 concurrent
created test:windows-vs2019-x64 #5088711
cmake windows shell vs2019 msvc-19.25 concurrent
 
  External
passed cdash

passed ghostflow-check-master

passed ghostflow-check-release

passed ghostflow-stager

passed cdash

passed cdash

passed cdash

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager