How to build only amd64 and avoid building gcc for 32 and x32?

[Svante Signell <svante.signell@gmail.com>]

Hi,

The title says it all. It is a little annoying to have to build gcc-x
for all multilib architectures. Where to set a build only for amd64 in
debian/rules* or DEB_BUILD_OPTIONS?

Trying to build gcc-8-8-20171128-1 fails with:
/usr/include/gnu/stubs.h:13:11: fatal error: gnu/stubs-x32.h: No such
file or directory
 # include <gnu/stubs-x32.h>

And I'm not the slightest interested in the 32|x32 builds!