travis: Switch to i386 version toolchain for x86

Currently this uses x86_64 version toolchain for x86 build in
travis-ci. Change it to i386 version to avoid updating the
buildman toolchain path every time when the toolchain version
number is changed, eg: from 7.3.0 to 8.1.0.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>