I need to build gcc RPMs from source including libgcc.i686 and
I don't want to run rpmbuild or mock twice with different arch argument.
One is long enough. Besides, the build directory already contains all
the needed files after %build phase:
The spec file from gcc-6.1.1-2.fc24.src.rpm suggests some support for
that. It has multilib_64_archs and multilib_32_arch macros, but I don't
know how to use them.