[CentOS] build c7 packages for i686 using mock

This might be an embarrassingly simple question.  How are people 
building multi-arch library packages on CentOS 7?  On older releases one 
could "mock -r epel-6-i386" and "mock -r epel-6-x86_64" to build a 
library twice.  With no 32-bit release, there's no epel-7-i386 target 
for mock.  Specifying --target=i386 or --arch-i386 both fail.

Do we have to set up the environment manually and rpm --rebuild inside 
the mock root?