add_library(): Allow imported object libraries with multi-arch

Fixes: #21276
42 jobs for !5771 with object-lib-multiarch in 124 minutes and 5 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Build
passed #5517967
linux docker build cmake manual
build:centos6-x86_64

00:04:26

passed #5517964
linux docker build cmake
build:debian10-iwyu

00:02:07

passed #5517968
docker build cmake linux-3.17 linux manual
build:fedora33-ninja

00:02:13

passed #5517966
linux-3.17 linux docker build cmake
build:fedora33-sphinx

00:02:12

passed #5517965
linux-3.17 linux docker build cmake
build:fedora33-tidy

00:05:22

passed #5517970
nonconcurrent xcode-12.3 shell macos cmake manual
build:macos-makefiles

00:02:09

passed #5517969
nonconcurrent xcode-12.3 shell macos cmake manual
build:macos-ninja

00:05:54

passed #5517971
nonconcurrent msvc-19.28 vs2019 shell windows cmake manual
build:windows-vs2019-x64-ninja

00:09:12

 
  Test
passed #5517972
x11 linux-3.17 linux docker cmake
test:fedora33-ninja

00:04:27

passed #5517974
nonconcurrent xcode-12.3 shell macos cmake
test:macos-makefiles

00:10:50

passed #5517973
nonconcurrent xcode-12.3 shell macos cmake
test:macos-ninja

00:09:11

passed #5522841
msvc-19.28 nonconcurrent vs2019 shell cmake windows
test:windows-vs2019-x64-ninja

00:25:40

failed #5522798
nonconcurrent vs2019 shell cmake windows msvc-19.28
test:windows-vs2019-x64-ninja

00:05:58

failed #5517975
nonconcurrent msvc-19.28 vs2019 shell windows cmake
test:windows-vs2019-x64-ninja

00:00:10

 
  Test Ext
passed #5517978
linux docker cuda-rt cmake
test:cuda10.2-nvidia

00:04:10

passed #5517976
linux-3.17 linux docker build cmake
test:debian10-ninja

00:05:21

passed #5517977
linux-3.17 linux docker build cmake
test:fedora33-makefiles

00:04:39

passed #5517979
linux-3.17 linux docker build cmake
test:fedora33-ninja-multi

00:02:51

passed #5517980
concurrent xcode-12.3 shell macos cmake
test:macos-xcode

00:55:11

passed #5517981
concurrent msvc-19.28 vs2019 shell windows cmake
test:windows-vs2019-x64

00:22:39

 
  External
passed cdash

passed ghostflow-check-master

passed ghostflow-stager

passed cdash

passed cdash

passed cdash

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager