Build 32bit Linux bundles on 64bit systems

As indicated in comment:2:ticket:25220 we might run into memory limitations when building 32bit Linux bundles on 32bit Linux systems. While this does not seem to be the case for a vanilla Tor Browser build yet, adding selfrando to the mix triggers this problem:

40:59.43 collect2: fatal error: ld terminated with signal 6 [Aborted]
40:59.43 compilation terminated.
40:59.43 /var/tmp/dist/binutils/bin/ld.gold.real: fatal error: libxul.so: mmap: failed to allocate 1457011852 bytes for output file: Cannot allocate memory
40:59.43 Linker execution failed, status: 1

We should move our compilation for 32bit Linux to a 64bit host.