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Building Older GCC Versions on x86-64 Debian/Ubuntu
Compiling 32-bit (i386) binaries in 64-bit (amd64) debian
apt-get install ia32-libs ia32-libs-gtk
gcc -m32 (see http://wiki.debian.org/DebianAMD64Faq)
linux32 make
Building an i386 kernel package on an amd64 machine without chroot
Cross-building for ARM in Ubuntu 64-bit
In addition to ia32-libs, install gcc-arm-linux-gnueabi (armel), or gcc-arm-linux-gnueabihf (armhf).
Using QEMU to create a rootfs, and build packages inside with the usual tools like pbuilder.