Test against x32 ABI on Travis

[omasanori]

After travis-ci/travis-ci#2046 is fixed, we will gain newer compilers, libraries and kernel on Travis, which means that we can test picrin against x32 ABI, a ILP32 processor-specific ABI for x86_64. We will get more chances to catch portability issues with it.

[nyuichi]

@omasanori

Beautiful. How can we add such architectures to the testing list? .travis.yml?

[omasanori]

Yes, just adding - CFLAGS="-mx32" to env in .travis.yml.

gcc-multilib has dependencies on libgcc and libc for x32 if supported, so no other changes will be required.