travis: Add 32 bit (-m32) cross-compile build

travis: Add 32 bit (-m32) cross-compile build

Inspired by Ben Pfaff's email on 32 bit build environment, this adds a
32bit build to the travis build matrix to catch alignment and padding
issues.

The 32 bit build is only enabled for non-DPDK builds as DPDK itself is
currently not capable to be compiled with -m32.

The build also has SSL disabled as the Ubuntu libssl-devel package is
not multiarch compatible on the travis build system.

Signed-off-by: Thomas Graf <tgraf@noironetworks.com>
Acked-by: Ben Pfaff <blp@nicira.com>